diff --git a/beacon_node/beacon_chain/src/metrics.rs b/beacon_node/beacon_chain/src/metrics.rs
index b52c4258f..d0f695062 100644
--- a/beacon_node/beacon_chain/src/metrics.rs
+++ b/beacon_node/beacon_chain/src/metrics.rs
@@ -874,6 +874,14 @@ lazy_static! {
         "beacon_sync_committee_message_gossip_verification_seconds",
         "Full runtime of sync contribution gossip verification"
     );
+    pub static ref SYNC_MESSAGE_EQUIVOCATIONS: Result<IntCounter> = try_create_int_counter(
+        "sync_message_equivocations_total",
+        "Number of sync messages with the same validator index for different blocks"
+    );
+    pub static ref SYNC_MESSAGE_EQUIVOCATIONS_TO_HEAD: Result<IntCounter> = try_create_int_counter(
+        "sync_message_equivocations_to_head_total",
+        "Number of sync message which conflict with a previous message but elect the head"
+    );
 
     /*
      * Sync Committee Contribution Verification
diff --git a/beacon_node/beacon_chain/src/observed_attesters.rs b/beacon_node/beacon_chain/src/observed_attesters.rs
index ed22beaec..59c67bd1b 100644
--- a/beacon_node/beacon_chain/src/observed_attesters.rs
+++ b/beacon_node/beacon_chain/src/observed_attesters.rs
@@ -20,7 +20,7 @@ use std::collections::{HashMap, HashSet};
 use std::hash::Hash;
 use std::marker::PhantomData;
 use types::slot_data::SlotData;
-use types::{Epoch, EthSpec, Slot, Unsigned};
+use types::{Epoch, EthSpec, Hash256, Slot, Unsigned};
 
 /// The maximum capacity of the `AutoPruningEpochContainer`.
 ///
@@ -39,10 +39,10 @@ pub const MAX_CACHED_EPOCHS: u64 = 3;
 
 pub type ObservedAttesters<E> = AutoPruningEpochContainer<EpochBitfield, E>;
 pub type ObservedSyncContributors<E> =
-    AutoPruningSlotContainer<SlotSubcommitteeIndex, SyncContributorSlotHashSet<E>, E>;
+    AutoPruningSlotContainer<SlotSubcommitteeIndex, Hash256, SyncContributorSlotHashSet<E>, E>;
 pub type ObservedAggregators<E> = AutoPruningEpochContainer<EpochHashSet, E>;
 pub type ObservedSyncAggregators<E> =
-    AutoPruningSlotContainer<SlotSubcommitteeIndex, SyncAggregatorSlotHashSet, E>;
+    AutoPruningSlotContainer<SlotSubcommitteeIndex, (), SyncAggregatorSlotHashSet, E>;
 
 #[derive(Debug, PartialEq)]
 pub enum Error {
@@ -62,7 +62,7 @@ pub enum Error {
 }
 
 /// Implemented on an item in an `AutoPruningContainer`.
-pub trait Item {
+pub trait Item<T> {
     /// Instantiate `Self` with the given `capacity`.
     fn with_capacity(capacity: usize) -> Self;
 
@@ -75,11 +75,11 @@ pub trait Item {
     /// Returns the number of validators that have been observed by `self`.
     fn validator_count(&self) -> usize;
 
-    /// Store `validator_index` in `self`.
-    fn insert(&mut self, validator_index: usize) -> bool;
+    /// Store `validator_index` and `value` in `self`.
+    fn insert(&mut self, validator_index: usize, value: T) -> bool;
 
-    /// Returns `true` if `validator_index` has been stored in `self`.
-    fn contains(&self, validator_index: usize) -> bool;
+    /// Returns `Some(T)` if there is an entry for `validator_index`.
+    fn get(&self, validator_index: usize) -> Option<T>;
 }
 
 /// Stores a `BitVec` that represents which validator indices have attested or sent sync committee
@@ -88,7 +88,7 @@ pub struct EpochBitfield {
     bitfield: BitVec,
 }
 
-impl Item for EpochBitfield {
+impl Item<()> for EpochBitfield {
     fn with_capacity(capacity: usize) -> Self {
         Self {
             bitfield: BitVec::with_capacity(capacity),
@@ -108,7 +108,7 @@ impl Item for EpochBitfield {
         self.bitfield.iter().filter(|bit| **bit).count()
     }
 
-    fn insert(&mut self, validator_index: usize) -> bool {
+    fn insert(&mut self, validator_index: usize, _value: ()) -> bool {
         self.bitfield
             .get_mut(validator_index)
             .map(|mut bit| {
@@ -129,8 +129,11 @@ impl Item for EpochBitfield {
             })
     }
 
-    fn contains(&self, validator_index: usize) -> bool {
-        self.bitfield.get(validator_index).map_or(false, |bit| *bit)
+    fn get(&self, validator_index: usize) -> Option<()> {
+        self.bitfield
+            .get(validator_index)
+            .map_or(false, |bit| *bit)
+            .then_some(())
     }
 }
 
@@ -140,7 +143,7 @@ pub struct EpochHashSet {
     set: HashSet<usize>,
 }
 
-impl Item for EpochHashSet {
+impl Item<()> for EpochHashSet {
     fn with_capacity(capacity: usize) -> Self {
         Self {
             set: HashSet::with_capacity(capacity),
@@ -163,27 +166,27 @@ impl Item for EpochHashSet {
 
     /// Inserts the `validator_index` in the set. Returns `true` if the `validator_index` was
     /// already in the set.
-    fn insert(&mut self, validator_index: usize) -> bool {
+    fn insert(&mut self, validator_index: usize, _value: ()) -> bool {
         !self.set.insert(validator_index)
     }
 
     /// Returns `true` if the `validator_index` is in the set.
-    fn contains(&self, validator_index: usize) -> bool {
-        self.set.contains(&validator_index)
+    fn get(&self, validator_index: usize) -> Option<()> {
+        self.set.contains(&validator_index).then_some(())
     }
 }
 
 /// Stores a `HashSet` of which validator indices have created a sync aggregate during a
 /// slot.
 pub struct SyncContributorSlotHashSet<E> {
-    set: HashSet<usize>,
+    map: HashMap<usize, Hash256>,
     phantom: PhantomData<E>,
 }
 
-impl<E: EthSpec> Item for SyncContributorSlotHashSet<E> {
+impl<E: EthSpec> Item<Hash256> for SyncContributorSlotHashSet<E> {
     fn with_capacity(capacity: usize) -> Self {
         Self {
-            set: HashSet::with_capacity(capacity),
+            map: HashMap::with_capacity(capacity),
             phantom: PhantomData,
         }
     }
@@ -194,22 +197,24 @@ impl<E: EthSpec> Item for SyncContributorSlotHashSet<E> {
     }
 
     fn len(&self) -> usize {
-        self.set.len()
+        self.map.len()
     }
 
     fn validator_count(&self) -> usize {
-        self.set.len()
+        self.map.len()
     }
 
     /// Inserts the `validator_index` in the set. Returns `true` if the `validator_index` was
     /// already in the set.
-    fn insert(&mut self, validator_index: usize) -> bool {
-        !self.set.insert(validator_index)
+    fn insert(&mut self, validator_index: usize, beacon_block_root: Hash256) -> bool {
+        self.map
+            .insert(validator_index, beacon_block_root)
+            .is_some()
     }
 
     /// Returns `true` if the `validator_index` is in the set.
-    fn contains(&self, validator_index: usize) -> bool {
-        self.set.contains(&validator_index)
+    fn get(&self, validator_index: usize) -> Option<Hash256> {
+        self.map.get(&validator_index).copied()
     }
 }
 
@@ -219,7 +224,7 @@ pub struct SyncAggregatorSlotHashSet {
     set: HashSet<usize>,
 }
 
-impl Item for SyncAggregatorSlotHashSet {
+impl Item<()> for SyncAggregatorSlotHashSet {
     fn with_capacity(capacity: usize) -> Self {
         Self {
             set: HashSet::with_capacity(capacity),
@@ -241,13 +246,13 @@ impl Item for SyncAggregatorSlotHashSet {
 
     /// Inserts the `validator_index` in the set. Returns `true` if the `validator_index` was
     /// already in the set.
-    fn insert(&mut self, validator_index: usize) -> bool {
+    fn insert(&mut self, validator_index: usize, _value: ()) -> bool {
         !self.set.insert(validator_index)
     }
 
     /// Returns `true` if the `validator_index` is in the set.
-    fn contains(&self, validator_index: usize) -> bool {
-        self.set.contains(&validator_index)
+    fn get(&self, validator_index: usize) -> Option<()> {
+        self.set.contains(&validator_index).then_some(())
     }
 }
 
@@ -275,7 +280,7 @@ impl<T, E: EthSpec> Default for AutoPruningEpochContainer<T, E> {
     }
 }
 
-impl<T: Item, E: EthSpec> AutoPruningEpochContainer<T, E> {
+impl<T: Item<()>, E: EthSpec> AutoPruningEpochContainer<T, E> {
     /// Observe that `validator_index` has produced attestation `a`. Returns `Ok(true)` if `a` has
     /// previously been observed for `validator_index`.
     ///
@@ -293,7 +298,7 @@ impl<T: Item, E: EthSpec> AutoPruningEpochContainer<T, E> {
         self.prune(epoch);
 
         if let Some(item) = self.items.get_mut(&epoch) {
-            Ok(item.insert(validator_index))
+            Ok(item.insert(validator_index, ()))
         } else {
             // To avoid re-allocations, try and determine a rough initial capacity for the new item
             // by obtaining the mean size of all items in earlier epoch.
@@ -309,7 +314,7 @@ impl<T: Item, E: EthSpec> AutoPruningEpochContainer<T, E> {
             let initial_capacity = sum.checked_div(count).unwrap_or_else(T::default_capacity);
 
             let mut item = T::with_capacity(initial_capacity);
-            item.insert(validator_index);
+            item.insert(validator_index, ());
             self.items.insert(epoch, item);
 
             Ok(false)
@@ -333,7 +338,7 @@ impl<T: Item, E: EthSpec> AutoPruningEpochContainer<T, E> {
         let exists = self
             .items
             .get(&epoch)
-            .map_or(false, |item| item.contains(validator_index));
+            .map_or(false, |item| item.get(validator_index).is_some());
 
         Ok(exists)
     }
@@ -392,7 +397,7 @@ impl<T: Item, E: EthSpec> AutoPruningEpochContainer<T, E> {
     pub fn index_seen_at_epoch(&self, index: usize, epoch: Epoch) -> bool {
         self.items
             .get(&epoch)
-            .map(|item| item.contains(index))
+            .map(|item| item.get(index).is_some())
             .unwrap_or(false)
     }
 }
@@ -405,23 +410,63 @@ impl<T: Item, E: EthSpec> AutoPruningEpochContainer<T, E> {
 /// sync contributions with an epoch prior to `data.slot - 3` will be cleared from the cache.
 ///
 /// `V` should be set to a `SyncAggregatorSlotHashSet` or a `SyncContributorSlotHashSet`.
-pub struct AutoPruningSlotContainer<K: SlotData + Eq + Hash, V, E: EthSpec> {
+pub struct AutoPruningSlotContainer<K: SlotData + Eq + Hash, S, V, E: EthSpec> {
     lowest_permissible_slot: Slot,
     items: HashMap<K, V>,
-    _phantom: PhantomData<E>,
+    _phantom_e: PhantomData<E>,
+    _phantom_s: PhantomData<S>,
 }
 
-impl<K: SlotData + Eq + Hash, V, E: EthSpec> Default for AutoPruningSlotContainer<K, V, E> {
+impl<K: SlotData + Eq + Hash, S, V, E: EthSpec> Default for AutoPruningSlotContainer<K, S, V, E> {
     fn default() -> Self {
         Self {
             lowest_permissible_slot: Slot::new(0),
             items: HashMap::new(),
-            _phantom: PhantomData,
+            _phantom_e: PhantomData,
+            _phantom_s: PhantomData,
         }
     }
 }
 
-impl<K: SlotData + Eq + Hash, V: Item, E: EthSpec> AutoPruningSlotContainer<K, V, E> {
+impl<K: SlotData + Eq + Hash + Copy, S, V: Item<S>, E: EthSpec>
+    AutoPruningSlotContainer<K, S, V, E>
+{
+    /// Observes the given `value` for the given `validator_index`.
+    ///
+    /// The `override_observation` function is supplied `previous_observation`
+    /// and `value`. If it returns `true`, then any existing observation will be
+    /// overridden.
+    ///
+    /// This function returns `Some` if:
+    /// - An observation already existed for the validator, AND,
+    /// - The `override_observation` function returned `false`.
+    ///
+    /// Alternatively, it returns `None` if:
+    /// - An observation did not already exist for the given validator, OR,
+    /// - The `override_observation` function returned `true`.
+    pub fn observe_validator_with_override<F>(
+        &mut self,
+        key: K,
+        validator_index: usize,
+        value: S,
+        override_observation: F,
+    ) -> Result<Option<S>, Error>
+    where
+        F: Fn(&S, &S) -> bool,
+    {
+        if let Some(prev_observation) = self.observation_for_validator(key, validator_index)? {
+            if override_observation(&prev_observation, &value) {
+                self.observe_validator(key, validator_index, value)?;
+                Ok(None)
+            } else {
+                Ok(Some(prev_observation))
+            }
+        } else {
+            self.observe_validator(key, validator_index, value)?;
+            Ok(None)
+        }
+    }
+
     /// Observe that `validator_index` has produced a sync committee message. Returns `Ok(true)` if
     /// the sync committee message  has previously been observed for `validator_index`.
     ///
@@ -429,14 +474,19 @@ impl<K: SlotData + Eq + Hash, V: Item, E: EthSpec> AutoPruningSlotContainer<K, V
     ///
     /// - `validator_index` is higher than `VALIDATOR_REGISTRY_LIMIT`.
     /// - `key.slot` is earlier than `self.lowest_permissible_slot`.
-    pub fn observe_validator(&mut self, key: K, validator_index: usize) -> Result<bool, Error> {
+    pub fn observe_validator(
+        &mut self,
+        key: K,
+        validator_index: usize,
+        value: S,
+    ) -> Result<bool, Error> {
         let slot = key.get_slot();
         self.sanitize_request(slot, validator_index)?;
 
         self.prune(slot);
 
         if let Some(item) = self.items.get_mut(&key) {
-            Ok(item.insert(validator_index))
+            Ok(item.insert(validator_index, value))
         } else {
             // To avoid re-allocations, try and determine a rough initial capacity for the new item
             // by obtaining the mean size of all items in earlier slot.
@@ -452,32 +502,45 @@ impl<K: SlotData + Eq + Hash, V: Item, E: EthSpec> AutoPruningSlotContainer<K, V
             let initial_capacity = sum.checked_div(count).unwrap_or_else(V::default_capacity);
 
             let mut item = V::with_capacity(initial_capacity);
-            item.insert(validator_index);
+            item.insert(validator_index, value);
             self.items.insert(key, item);
 
             Ok(false)
         }
     }
 
-    /// Returns `Ok(true)` if the `validator_index` has already produced a conflicting sync committee message.
-    ///
-    /// ## Errors
-    ///
-    /// - `validator_index` is higher than `VALIDATOR_REGISTRY_LIMIT`.
-    /// - `key.slot` is earlier than `self.lowest_permissible_slot`.
+    // Identical to `Self::observation_for_validator` but discards the
+    // observation, simply returning `true` if the validator has been observed
+    // at all.
     pub fn validator_has_been_observed(
         &self,
         key: K,
         validator_index: usize,
     ) -> Result<bool, Error> {
+        self.observation_for_validator(key, validator_index)
+            .map(|observation| observation.is_some())
+    }
+
+    /// Returns `Ok(Some)` if the `validator_index` has already produced a
+    /// conflicting sync committee message.
+    ///
+    /// ## Errors
+    ///
+    /// - `validator_index` is higher than `VALIDATOR_REGISTRY_LIMIT`.
+    /// - `key.slot` is earlier than `self.lowest_permissible_slot`.
+    pub fn observation_for_validator(
+        &self,
+        key: K,
+        validator_index: usize,
+    ) -> Result<Option<S>, Error> {
         self.sanitize_request(key.get_slot(), validator_index)?;
 
-        let exists = self
+        let observation = self
             .items
             .get(&key)
-            .map_or(false, |item| item.contains(validator_index));
+            .and_then(|item| item.get(validator_index));
 
-        Ok(exists)
+        Ok(observation)
     }
 
     /// Returns the number of validators that have been observed at the given `slot`. Returns
@@ -561,6 +624,116 @@ mod tests {
 
     type E = types::MainnetEthSpec;
 
+    #[test]
+    fn value_storage() {
+        type Container = AutoPruningSlotContainer<Slot, Hash256, SyncContributorSlotHashSet<E>, E>;
+
+        let mut store: Container = <_>::default();
+        let key = Slot::new(0);
+        let validator_index = 0;
+        let value = Hash256::zero();
+
+        // Assert there is no entry.
+        assert!(store
+            .observation_for_validator(key, validator_index)
+            .unwrap()
+            .is_none());
+        assert!(!store
+            .validator_has_been_observed(key, validator_index)
+            .unwrap());
+
+        // Add an entry.
+        assert!(!store
+            .observe_validator(key, validator_index, value)
+            .unwrap());
+
+        // Assert there is a correct entry.
+        assert_eq!(
+            store
+                .observation_for_validator(key, validator_index)
+                .unwrap(),
+            Some(value)
+        );
+        assert!(store
+            .validator_has_been_observed(key, validator_index)
+            .unwrap());
+
+        let alternate_value = Hash256::from_low_u64_be(1);
+
+        // Assert that override false does not override.
+        assert_eq!(
+            store
+                .observe_validator_with_override(key, validator_index, alternate_value, |_, _| {
+                    false
+                })
+                .unwrap(),
+            Some(value)
+        );
+
+        // Assert that override true overrides and acts as if there was never an
+        // entry there.
+        assert_eq!(
+            store
+                .observe_validator_with_override(key, validator_index, alternate_value, |_, _| {
+                    true
+                })
+                .unwrap(),
+            None
+        );
+        assert_eq!(
+            store
+                .observation_for_validator(key, validator_index)
+                .unwrap(),
+            Some(alternate_value)
+        );
+
+        // Reset the store.
+        let mut store: Container = <_>::default();
+
+        // Asset that a new entry with override = false is inserted
+        assert_eq!(
+            store
+                .observation_for_validator(key, validator_index)
+                .unwrap(),
+            None
+        );
+        assert_eq!(
+            store
+                .observe_validator_with_override(key, validator_index, value, |_, _| { false })
+                .unwrap(),
+            None,
+        );
+        assert_eq!(
+            store
+                .observation_for_validator(key, validator_index)
+                .unwrap(),
+            Some(value)
+        );
+
+        // Reset the store.
+        let mut store: Container = <_>::default();
+
+        // Asset that a new entry with override = true is inserted
+        assert_eq!(
+            store
+                .observation_for_validator(key, validator_index)
+                .unwrap(),
+            None
+        );
+        assert_eq!(
+            store
+                .observe_validator_with_override(key, validator_index, value, |_, _| { true })
+                .unwrap(),
+            None,
+        );
+        assert_eq!(
+            store
+                .observation_for_validator(key, validator_index)
+                .unwrap(),
+            Some(value)
+        );
+    }
+
     macro_rules! test_suite_epoch {
         ($mod_name: ident, $type: ident) => {
             #[cfg(test)]
@@ -722,7 +895,7 @@ mod tests {
     test_suite_epoch!(observed_aggregators, ObservedAggregators);
 
     macro_rules! test_suite_slot {
-        ($mod_name: ident, $type: ident) => {
+        ($mod_name: ident, $type: ident, $value: expr) => {
             #[cfg(test)]
             mod $mod_name {
                 use super::*;
@@ -737,7 +910,7 @@ mod tests {
                             "should indicate an unknown item is unknown"
                         );
                         assert_eq!(
-                            store.observe_validator(key, i),
+                            store.observe_validator(key, i, $value),
                             Ok(false),
                             "should observe new item"
                         );
@@ -750,7 +923,7 @@ mod tests {
                             "should indicate a known item is known"
                         );
                         assert_eq!(
-                            store.observe_validator(key, i),
+                            store.observe_validator(key, i, $value),
                             Ok(true),
                             "should acknowledge an existing item"
                         );
@@ -997,6 +1170,10 @@ mod tests {
             }
         };
     }
-    test_suite_slot!(observed_sync_contributors, ObservedSyncContributors);
-    test_suite_slot!(observed_sync_aggregators, ObservedSyncAggregators);
+    test_suite_slot!(
+        observed_sync_contributors,
+        ObservedSyncContributors,
+        Hash256::zero()
+    );
+    test_suite_slot!(observed_sync_aggregators, ObservedSyncAggregators, ());
 }
diff --git a/beacon_node/beacon_chain/src/sync_committee_verification.rs b/beacon_node/beacon_chain/src/sync_committee_verification.rs
index 4b4228e71..14cdc2400 100644
--- a/beacon_node/beacon_chain/src/sync_committee_verification.rs
+++ b/beacon_node/beacon_chain/src/sync_committee_verification.rs
@@ -153,7 +153,21 @@ pub enum Error {
     /// It's unclear if this sync message is valid, however we have already observed a
     /// signature from this validator for this slot and should not observe
     /// another.
-    PriorSyncCommitteeMessageKnown { validator_index: u64, slot: Slot },
+    PriorSyncCommitteeMessageKnown {
+        validator_index: u64,
+        slot: Slot,
+        prev_root: Hash256,
+        new_root: Hash256,
+    },
+    /// We have already observed a contribution for the aggregator and refuse to
+    /// process another.
+    ///
+    /// ## Peer scoring
+    ///
+    /// It's unclear if this sync message is valid, however we have already observed a
+    /// signature from this validator for this slot and should not observe
+    /// another.
+    PriorSyncContributionMessageKnown { validator_index: u64, slot: Slot },
     /// The sync committee message was received on an invalid sync committee message subnet.
     ///
     /// ## Peer scoring
@@ -378,10 +392,10 @@ impl<T: BeaconChainTypes> VerifiedSyncContribution<T> {
         if chain
             .observed_sync_aggregators
             .write()
-            .observe_validator(observed_key, aggregator_index as usize)
+            .observe_validator(observed_key, aggregator_index as usize, ())
             .map_err(BeaconChainError::from)?
         {
-            return Err(Error::PriorSyncCommitteeMessageKnown {
+            return Err(Error::PriorSyncContributionMessageKnown {
                 validator_index: aggregator_index,
                 slot: contribution.slot,
             });
@@ -450,19 +464,40 @@ impl VerifiedSyncCommitteeMessage {
         // The sync committee message is the first valid message received for the participating validator
         // for the slot, sync_message.slot.
         let validator_index = sync_message.validator_index;
-        if chain
+        let head_root = chain.canonical_head.cached_head().head_block_root();
+        let new_root = sync_message.beacon_block_root;
+        let should_override_prev = |prev_root: &Hash256, new_root: &Hash256| {
+            let roots_differ = new_root != prev_root;
+            let new_elects_head = new_root == &head_root;
+
+            if roots_differ {
+                // Track sync committee messages that differ from each other.
+                metrics::inc_counter(&metrics::SYNC_MESSAGE_EQUIVOCATIONS);
+                if new_elects_head {
+                    // Track sync committee messages that swap from an old block to a new block.
+                    metrics::inc_counter(&metrics::SYNC_MESSAGE_EQUIVOCATIONS_TO_HEAD);
+                }
+            }
+
+            roots_differ && new_elects_head
+        };
+        if let Some(prev_root) = chain
             .observed_sync_contributors
             .read()
-            .validator_has_been_observed(
+            .observation_for_validator(
                 SlotSubcommitteeIndex::new(sync_message.slot, subnet_id.into()),
                 validator_index as usize,
             )
             .map_err(BeaconChainError::from)?
         {
-            return Err(Error::PriorSyncCommitteeMessageKnown {
-                validator_index,
-                slot: sync_message.slot,
-            });
+            if !should_override_prev(&prev_root, &new_root) {
+                return Err(Error::PriorSyncCommitteeMessageKnown {
+                    validator_index,
+                    slot: sync_message.slot,
+                    prev_root,
+                    new_root,
+                });
+            }
         }
 
         // The aggregate signature of the sync committee message is valid.
@@ -474,18 +509,22 @@ impl VerifiedSyncCommitteeMessage {
         // It's important to double check that the sync committee message still hasn't been observed, since
         // there can be a race-condition if we receive two sync committee messages at the same time and
         // process them in different threads.
-        if chain
+        if let Some(prev_root) = chain
             .observed_sync_contributors
             .write()
-            .observe_validator(
+            .observe_validator_with_override(
                 SlotSubcommitteeIndex::new(sync_message.slot, subnet_id.into()),
                 validator_index as usize,
+                sync_message.beacon_block_root,
+                should_override_prev,
             )
             .map_err(BeaconChainError::from)?
         {
             return Err(Error::PriorSyncCommitteeMessageKnown {
                 validator_index,
                 slot: sync_message.slot,
+                prev_root,
+                new_root,
             });
         }
 
diff --git a/beacon_node/beacon_chain/tests/sync_committee_verification.rs b/beacon_node/beacon_chain/tests/sync_committee_verification.rs
index 239f55e7d..4204a5121 100644
--- a/beacon_node/beacon_chain/tests/sync_committee_verification.rs
+++ b/beacon_node/beacon_chain/tests/sync_committee_verification.rs
@@ -5,12 +5,16 @@ use beacon_chain::test_utils::{BeaconChainHarness, EphemeralHarnessType, Relativ
 use int_to_bytes::int_to_bytes32;
 use lazy_static::lazy_static;
 use safe_arith::SafeArith;
+use state_processing::{
+    per_block_processing::{altair::sync_committee::process_sync_aggregate, VerifySignatures},
+    state_advance::complete_state_advance,
+};
 use store::{SignedContributionAndProof, SyncCommitteeMessage};
 use tree_hash::TreeHash;
 use types::consts::altair::SYNC_COMMITTEE_SUBNET_COUNT;
 use types::{
     AggregateSignature, Epoch, EthSpec, Hash256, Keypair, MainnetEthSpec, SecretKey, Slot,
-    SyncSelectionProof, SyncSubnetId, Unsigned,
+    SyncContributionData, SyncSelectionProof, SyncSubnetId, Unsigned,
 };
 
 pub type E = MainnetEthSpec;
@@ -47,10 +51,29 @@ fn get_valid_sync_committee_message(
     relative_sync_committee: RelativeSyncCommittee,
     message_index: usize,
 ) -> (SyncCommitteeMessage, usize, SecretKey, SyncSubnetId) {
-    let head_state = harness.chain.head_beacon_state_cloned();
     let head_block_root = harness.chain.head_snapshot().beacon_block_root;
+    get_valid_sync_committee_message_for_block(
+        harness,
+        slot,
+        relative_sync_committee,
+        message_index,
+        head_block_root,
+    )
+}
+
+/// Returns a sync message that is valid for some slot in the given `chain`.
+///
+/// Also returns some info about who created it.
+fn get_valid_sync_committee_message_for_block(
+    harness: &BeaconChainHarness<EphemeralHarnessType<E>>,
+    slot: Slot,
+    relative_sync_committee: RelativeSyncCommittee,
+    message_index: usize,
+    block_root: Hash256,
+) -> (SyncCommitteeMessage, usize, SecretKey, SyncSubnetId) {
+    let head_state = harness.chain.head_beacon_state_cloned();
     let (signature, _) = harness
-        .make_sync_committee_messages(&head_state, head_block_root, slot, relative_sync_committee)
+        .make_sync_committee_messages(&head_state, block_root, slot, relative_sync_committee)
         .get(0)
         .expect("sync messages should exist")
         .get(message_index)
@@ -119,7 +142,7 @@ fn get_non_aggregator(
             subcommittee.iter().find_map(|pubkey| {
                 let validator_index = harness
                     .chain
-                    .validator_index(&pubkey)
+                    .validator_index(pubkey)
                     .expect("should get validator index")
                     .expect("pubkey should exist in beacon chain");
 
@@ -376,7 +399,7 @@ async fn aggregated_gossip_verification() {
         SyncCommitteeError::AggregatorNotInCommittee {
             aggregator_index
         }
-        if aggregator_index == valid_aggregate.message.aggregator_index as u64
+        if aggregator_index == valid_aggregate.message.aggregator_index
     );
 
     /*
@@ -472,7 +495,7 @@ async fn aggregated_gossip_verification() {
 
     assert_invalid!(
         "sync contribution created with incorrect sync committee",
-        next_valid_contribution.clone(),
+        next_valid_contribution,
         SyncCommitteeError::InvalidSignature | SyncCommitteeError::AggregatorNotInCommittee { .. }
     );
 }
@@ -496,6 +519,30 @@ async fn unaggregated_gossip_verification() {
 
     let (valid_sync_committee_message, expected_validator_index, validator_sk, subnet_id) =
         get_valid_sync_committee_message(&harness, current_slot, RelativeSyncCommittee::Current, 0);
+    let parent_root = harness.chain.head_snapshot().beacon_block.parent_root();
+    let (valid_sync_committee_message_to_parent, _, _, _) =
+        get_valid_sync_committee_message_for_block(
+            &harness,
+            current_slot,
+            RelativeSyncCommittee::Current,
+            0,
+            parent_root,
+        );
+
+    assert_eq!(
+        valid_sync_committee_message.slot, valid_sync_committee_message_to_parent.slot,
+        "test pre-condition: same slot"
+    );
+    assert_eq!(
+        valid_sync_committee_message.validator_index,
+        valid_sync_committee_message_to_parent.validator_index,
+        "test pre-condition: same validator index"
+    );
+    assert!(
+        valid_sync_committee_message.beacon_block_root
+            != valid_sync_committee_message_to_parent.beacon_block_root,
+        "test pre-condition: differing roots"
+    );
 
     macro_rules! assert_invalid {
             ($desc: tt, $attn_getter: expr, $subnet_getter: expr, $($error: pat_param) |+ $( if $guard: expr )?) => {
@@ -602,28 +649,130 @@ async fn unaggregated_gossip_verification() {
         SyncCommitteeError::InvalidSignature
     );
 
+    let head_root = valid_sync_committee_message.beacon_block_root;
+    let parent_root = valid_sync_committee_message_to_parent.beacon_block_root;
+
+    let verifed_message_to_parent = harness
+        .chain
+        .verify_sync_committee_message_for_gossip(
+            valid_sync_committee_message_to_parent.clone(),
+            subnet_id,
+        )
+        .expect("valid sync message to parent should be verified");
+    // Add the aggregate to the pool.
     harness
         .chain
-        .verify_sync_committee_message_for_gossip(valid_sync_committee_message.clone(), subnet_id)
-        .expect("valid sync message should be verified");
+        .add_to_naive_sync_aggregation_pool(verifed_message_to_parent)
+        .unwrap();
 
     /*
      * The following test ensures that:
      *
-     * There has been no other valid sync committee message for the declared slot for the
-     * validator referenced by sync_committee_message.validator_index.
+     * A sync committee message from the same validator to the same block will
+     * be rejected.
      */
     assert_invalid!(
-        "sync message that has already been seen",
-        valid_sync_committee_message,
+        "sync message to parent block that has already been seen",
+        valid_sync_committee_message_to_parent.clone(),
         subnet_id,
         SyncCommitteeError::PriorSyncCommitteeMessageKnown {
             validator_index,
             slot,
+            prev_root,
+            new_root
         }
-        if validator_index == expected_validator_index as u64 && slot == current_slot
+        if validator_index == expected_validator_index as u64 && slot == current_slot && prev_root == parent_root && new_root == parent_root
     );
 
+    let verified_message_to_head = harness
+        .chain
+        .verify_sync_committee_message_for_gossip(valid_sync_committee_message.clone(), subnet_id)
+        .expect("valid sync message to the head should be verified");
+    // Add the aggregate to the pool.
+    harness
+        .chain
+        .add_to_naive_sync_aggregation_pool(verified_message_to_head)
+        .unwrap();
+
+    /*
+     * The following test ensures that:
+     *
+     * A sync committee message from the same validator to the same block will
+     * be rejected.
+     */
+    assert_invalid!(
+        "sync message to the head that has already been seen",
+        valid_sync_committee_message.clone(),
+        subnet_id,
+        SyncCommitteeError::PriorSyncCommitteeMessageKnown {
+            validator_index,
+            slot,
+            prev_root,
+            new_root
+        }
+        if validator_index == expected_validator_index as u64 && slot == current_slot && prev_root == head_root && new_root == head_root
+    );
+
+    /*
+     * The following test ensures that:
+     *
+     * A sync committee message from the same validator to a non-head block will
+     * be rejected.
+     */
+    assert_invalid!(
+        "sync message to parent after message to head has already been seen",
+        valid_sync_committee_message_to_parent.clone(),
+        subnet_id,
+        SyncCommitteeError::PriorSyncCommitteeMessageKnown {
+            validator_index,
+            slot,
+            prev_root,
+            new_root
+        }
+        if validator_index == expected_validator_index as u64 && slot == current_slot && prev_root == head_root && new_root == parent_root
+    );
+
+    // Ensure that the sync aggregates in the op pool for both the parent block and head block are valid.
+    let chain = &harness.chain;
+    let check_sync_aggregate = |root: Hash256| async move {
+        // Generate an aggregate sync message from the naive aggregation pool.
+        let aggregate = chain
+            .get_aggregated_sync_committee_contribution(&SyncContributionData {
+                // It's a test pre-condition that both sync messages have the same slot.
+                slot: valid_sync_committee_message.slot,
+                beacon_block_root: root,
+                subcommittee_index: subnet_id.into(),
+            })
+            .unwrap()
+            .unwrap();
+
+        // Insert the aggregate into the op pool.
+        chain.op_pool.insert_sync_contribution(aggregate).unwrap();
+
+        // Load the block and state for the given root.
+        let block = chain.get_block(&root).await.unwrap().unwrap();
+        let mut state = chain.get_state(&block.state_root(), None).unwrap().unwrap();
+
+        // Advance the state to simulate a pre-state for block production.
+        let slot = valid_sync_committee_message.slot + 1;
+        complete_state_advance(&mut state, Some(block.state_root()), slot, &chain.spec).unwrap();
+
+        // Get an aggregate that would be included in a block.
+        let aggregate_for_inclusion = chain.op_pool.get_sync_aggregate(&state).unwrap().unwrap();
+
+        // Validate the retrieved aggregate against the state.
+        process_sync_aggregate(
+            &mut state,
+            &aggregate_for_inclusion,
+            0,
+            VerifySignatures::True,
+            &chain.spec,
+        )
+        .unwrap();
+    };
+    check_sync_aggregate(valid_sync_committee_message.beacon_block_root).await;
+    check_sync_aggregate(valid_sync_committee_message_to_parent.beacon_block_root).await;
+
     /*
      * The following test ensures that:
      *
@@ -649,7 +798,7 @@ async fn unaggregated_gossip_verification() {
 
     assert_invalid!(
         "sync message on incorrect subnet",
-        next_valid_sync_committee_message.clone(),
+        next_valid_sync_committee_message,
         next_subnet_id,
         SyncCommitteeError::InvalidSubnetId {
             received,
diff --git a/beacon_node/http_api/src/sync_committees.rs b/beacon_node/http_api/src/sync_committees.rs
index a6acf308f..c728fbeb1 100644
--- a/beacon_node/http_api/src/sync_committees.rs
+++ b/beacon_node/http_api/src/sync_committees.rs
@@ -199,10 +199,14 @@ pub fn process_sync_committee_signatures<T: BeaconChainTypes>(
                 Err(SyncVerificationError::PriorSyncCommitteeMessageKnown {
                     validator_index,
                     slot,
+                    prev_root,
+                    new_root,
                 }) => {
                     debug!(
                         log,
                         "Ignoring already-known sync message";
+                        "new_root" => ?new_root,
+                        "prev_root" => ?prev_root,
                         "slot" => slot,
                         "validator_index" => validator_index,
                     );
diff --git a/beacon_node/network/src/beacon_processor/worker/gossip_methods.rs b/beacon_node/network/src/beacon_processor/worker/gossip_methods.rs
index 9d85bc545..cb4533f5a 100644
--- a/beacon_node/network/src/beacon_processor/worker/gossip_methods.rs
+++ b/beacon_node/network/src/beacon_processor/worker/gossip_methods.rs
@@ -2343,6 +2343,25 @@ impl<T: BeaconChainTypes> Worker<T> {
                     "peer_id" => %peer_id,
                     "type" => ?message_type,
                 );
+
+                // Do not penalize the peer.
+
+                self.propagate_validation_result(message_id, peer_id, MessageAcceptance::Ignore);
+
+                return;
+            }
+            SyncCommitteeError::PriorSyncContributionMessageKnown { .. } => {
+                /*
+                 * We have already seen a sync contribution message from this validator for this epoch.
+                 *
+                 * The peer is not necessarily faulty.
+                 */
+                debug!(
+                    self.log,
+                    "Prior sync contribution message known";
+                    "peer_id" => %peer_id,
+                    "type" => ?message_type,
+                );
                 // We still penalize the peer slightly. We don't want this to be a recurring
                 // behaviour.
                 self.gossip_penalize_peer(