diff --git a/beacon_node/beacon_chain/src/beacon_chain.rs b/beacon_node/beacon_chain/src/beacon_chain.rs
index 83b593f9b..fe0740fa2 100644
--- a/beacon_node/beacon_chain/src/beacon_chain.rs
+++ b/beacon_node/beacon_chain/src/beacon_chain.rs
@@ -406,7 +406,7 @@ pub struct BeaconChain<T: BeaconChainTypes> {
     /// Maintains a record of which validators have proposed blocks for each slot.
     pub observed_block_producers: RwLock<ObservedBlockProducers<T::EthSpec>>,
     /// Maintains a record of blob sidecars seen over the gossip network.
-    pub(crate) observed_blob_sidecars: RwLock<ObservedBlobSidecars<T::EthSpec>>,
+    pub observed_blob_sidecars: RwLock<ObservedBlobSidecars<T::EthSpec>>,
     /// Maintains a record of which validators have submitted voluntary exits.
     pub(crate) observed_voluntary_exits: Mutex<ObservedOperations<SignedVoluntaryExit, T::EthSpec>>,
     /// Maintains a record of which validators we've seen proposer slashings for.
diff --git a/beacon_node/beacon_chain/src/blob_verification.rs b/beacon_node/beacon_chain/src/blob_verification.rs
index cc087e74a..a4750fd6e 100644
--- a/beacon_node/beacon_chain/src/blob_verification.rs
+++ b/beacon_node/beacon_chain/src/blob_verification.rs
@@ -420,7 +420,7 @@ pub fn validate_blob_sidecar_for_gossip<T: BeaconChainTypes>(
     if chain
         .observed_blob_sidecars
         .read()
-        .is_known(&blob_sidecar)
+        .proposer_is_known(&blob_sidecar)
         .map_err(|e| GossipBlobError::BeaconChainError(e.into()))?
     {
         return Err(GossipBlobError::RepeatBlob {
diff --git a/beacon_node/beacon_chain/src/observed_blob_sidecars.rs b/beacon_node/beacon_chain/src/observed_blob_sidecars.rs
index 4f8496144..488faf12a 100644
--- a/beacon_node/beacon_chain/src/observed_blob_sidecars.rs
+++ b/beacon_node/beacon_chain/src/observed_blob_sidecars.rs
@@ -3,9 +3,10 @@
 //! Only `BlobSidecar`s that have completed proposer signature verification can be added
 //! to this cache to reduce DoS risks.
 
+use crate::observed_block_producers::{ProposalKey, SeenBlock};
 use std::collections::{HashMap, HashSet};
 use std::marker::PhantomData;
-use types::{BlobSidecar, EthSpec, Slot};
+use types::{BlobSidecar, EthSpec, Hash256, Slot};
 
 #[derive(Debug, PartialEq)]
 pub enum Error {
@@ -29,7 +30,7 @@ pub enum Error {
 pub struct ObservedBlobSidecars<T: EthSpec> {
     finalized_slot: Slot,
     /// Stores all received blob indices for a given `(ValidatorIndex, Slot)` tuple.
-    items: HashMap<(u64, Slot), HashSet<u64>>,
+    items: HashMap<ProposalKey, (HashSet<u64>, HashSet<Hash256>)>,
     _phantom: PhantomData<T>,
 }
 
@@ -50,27 +51,74 @@ impl<T: EthSpec> ObservedBlobSidecars<T> {
     ///
     /// The supplied `blob_sidecar` **MUST** have completed proposer signature verification.
     pub fn observe_sidecar(&mut self, blob_sidecar: &BlobSidecar<T>) -> Result<bool, Error> {
+        let block_root = blob_sidecar.block_root();
         self.sanitize_blob_sidecar(blob_sidecar)?;
 
-        let did_not_exist = self
+        let (blob_indices, block_roots) = self
             .items
-            .entry((blob_sidecar.block_proposer_index(), blob_sidecar.slot()))
-            .or_insert_with(|| HashSet::with_capacity(T::max_blobs_per_block()))
-            .insert(blob_sidecar.index);
+            .entry(ProposalKey {
+                slot: blob_sidecar.slot(),
+                proposer: blob_sidecar.block_proposer_index(),
+            })
+            .or_insert_with(|| {
+                (
+                    HashSet::with_capacity(T::max_blobs_per_block()),
+                    HashSet::new(),
+                )
+            });
+        let did_not_exist = blob_indices.insert(blob_sidecar.index);
+        block_roots.insert(block_root);
 
         Ok(!did_not_exist)
     }
 
     /// Returns `true` if the `blob_sidecar` has already been observed in the cache within the prune window.
-    pub fn is_known(&self, blob_sidecar: &BlobSidecar<T>) -> Result<bool, Error> {
+    pub fn proposer_is_known(&self, blob_sidecar: &BlobSidecar<T>) -> Result<bool, Error> {
         self.sanitize_blob_sidecar(blob_sidecar)?;
         let is_known = self
             .items
-            .get(&(blob_sidecar.block_proposer_index(), blob_sidecar.slot()))
-            .map_or(false, |set| set.contains(&blob_sidecar.index));
+            .get(&ProposalKey {
+                slot: blob_sidecar.slot(),
+                proposer: blob_sidecar.block_proposer_index(),
+            })
+            .map_or(false, |(blob_indices, _block_roots)| {
+                blob_indices.contains(&blob_sidecar.index)
+            });
         Ok(is_known)
     }
 
+    /// Returns `Ok(true)` if the `block_root` has been observed in a blob sidecar message before, `Ok(false)` if not.
+    /// Does not update the cache, so calling this function multiple times will continue to return
+    /// `Ok(false)`, until `Self::observe_proposer` is called.
+    ///
+    /// ## Errors
+    ///
+    /// - `key.proposer_index` is greater than `VALIDATOR_REGISTRY_LIMIT`.
+    /// - `key.slot` is equal to or less than the latest pruned `finalized_slot`.
+    pub fn proposer_has_been_observed(
+        &self,
+        slot: Slot,
+        proposer: u64,
+        block_root: Hash256,
+    ) -> Result<SeenBlock, Error> {
+        let key = ProposalKey { slot, proposer };
+        if let Some((_, block_roots)) = self.items.get(&key) {
+            let block_already_known = block_roots.contains(&block_root);
+            let no_prev_known_blocks =
+                block_roots.difference(&HashSet::from([block_root])).count() == 0;
+
+            if !no_prev_known_blocks {
+                Ok(SeenBlock::Slashable)
+            } else if block_already_known {
+                Ok(SeenBlock::Duplicate)
+            } else {
+                Ok(SeenBlock::UniqueNonSlashable)
+            }
+        } else {
+            Ok(SeenBlock::UniqueNonSlashable)
+        }
+    }
+
     fn sanitize_blob_sidecar(&self, blob_sidecar: &BlobSidecar<T>) -> Result<(), Error> {
         if blob_sidecar.index >= T::max_blobs_per_block() as u64 {
             return Err(Error::InvalidBlobIndex(blob_sidecar.index));
@@ -93,7 +141,7 @@ impl<T: EthSpec> ObservedBlobSidecars<T> {
         }
 
         self.finalized_slot = finalized_slot;
-        self.items.retain(|k, _| k.1 > finalized_slot);
+        self.items.retain(|k, _| k.slot > finalized_slot);
     }
 }
 
@@ -140,14 +188,20 @@ mod tests {
             1,
             "only one (validator_index, slot) tuple should be present"
         );
+
+        let (cached_blob_indices, cached_block_roots) = cache
+            .items
+            .get(&ProposalKey::new(proposer_index_a, Slot::new(0)))
+            .expect("slot zero should be present");
         assert_eq!(
-            cache
-                .items
-                .get(&(proposer_index_a, Slot::new(0)))
-                .expect("slot zero should be present")
-                .len(),
+            cached_blob_indices.len(),
             1,
-            "only one item should be present"
+            "only one proposer should be present"
+        );
+        assert_eq!(
+            cached_block_roots.len(),
+            1,
+            "only one block root should be present"
         );
 
         /*
@@ -158,14 +212,19 @@ mod tests {
 
         assert_eq!(cache.finalized_slot, 0, "finalized slot is zero");
         assert_eq!(cache.items.len(), 1, "only one slot should be present");
+        let (cached_blob_indices, cached_block_roots) = cache
+            .items
+            .get(&ProposalKey::new(proposer_index_a, Slot::new(0)))
+            .expect("slot zero should be present");
         assert_eq!(
-            cache
-                .items
-                .get(&(proposer_index_a, Slot::new(0)))
-                .expect("slot zero should be present")
-                .len(),
+            cached_blob_indices.len(),
             1,
-            "only one item should be present"
+            "only one proposer should be present"
+        );
+        assert_eq!(
+            cached_block_roots.len(),
+            1,
+            "only one block root should be present"
         );
 
         /*
@@ -215,15 +274,20 @@ mod tests {
         );
 
         assert_eq!(cache.items.len(), 1, "only one slot should be present");
+        let (cached_blob_indices, cached_block_roots) = cache
+            .items
+            .get(&ProposalKey::new(proposer_index_b, Slot::new(three_epochs)))
+            .expect("the three epochs slot should be present");
         assert_eq!(
-            cache
-                .items
-                .get(&(proposer_index_b, Slot::new(three_epochs)))
-                .expect("the three epochs slot should be present")
-                .len(),
+            cached_blob_indices.len(),
             1,
             "only one proposer should be present"
         );
+        assert_eq!(
+            cached_block_roots.len(),
+            1,
+            "only one block root should be present"
+        );
 
         /*
          * Check that a prune doesnt wipe later blocks
@@ -239,15 +303,20 @@ mod tests {
         );
 
         assert_eq!(cache.items.len(), 1, "only one slot should be present");
+        let (cached_blob_indices, cached_block_roots) = cache
+            .items
+            .get(&ProposalKey::new(proposer_index_b, Slot::new(three_epochs)))
+            .expect("the three epochs slot should be present");
         assert_eq!(
-            cache
-                .items
-                .get(&(proposer_index_b, Slot::new(three_epochs)))
-                .expect("the three epochs slot should be present")
-                .len(),
+            cached_blob_indices.len(),
             1,
             "only one proposer should be present"
         );
+        assert_eq!(
+            cached_block_roots.len(),
+            1,
+            "only one block root should be present"
+        );
     }
 
     #[test]
@@ -259,7 +328,7 @@ mod tests {
         let sidecar_a = get_blob_sidecar(0, proposer_index_a, 0);
 
         assert_eq!(
-            cache.is_known(&sidecar_a),
+            cache.proposer_is_known(&sidecar_a),
             Ok(false),
             "no observation in empty cache"
         );
@@ -271,7 +340,7 @@ mod tests {
         );
 
         assert_eq!(
-            cache.is_known(&sidecar_a),
+            cache.proposer_is_known(&sidecar_a),
             Ok(true),
             "observed block is indicated as true"
         );
@@ -284,15 +353,20 @@ mod tests {
 
         assert_eq!(cache.finalized_slot, 0, "finalized slot is zero");
         assert_eq!(cache.items.len(), 1, "only one slot should be present");
+        let (cached_blob_indices, cached_block_roots) = cache
+            .items
+            .get(&ProposalKey::new(proposer_index_a, Slot::new(0)))
+            .expect("slot zero should be present");
         assert_eq!(
-            cache
-                .items
-                .get(&(proposer_index_a, Slot::new(0)))
-                .expect("slot zero should be present")
-                .len(),
+            cached_blob_indices.len(),
             1,
             "only one proposer should be present"
         );
+        assert_eq!(
+            cached_block_roots.len(),
+            1,
+            "only one block root should be present"
+        );
 
         // Slot 1, proposer 0
 
@@ -300,7 +374,7 @@ mod tests {
         let sidecar_b = get_blob_sidecar(1, proposer_index_b, 0);
 
         assert_eq!(
-            cache.is_known(&sidecar_b),
+            cache.proposer_is_known(&sidecar_b),
             Ok(false),
             "no observation for new slot"
         );
@@ -310,7 +384,7 @@ mod tests {
             "can observe proposer for new slot, indicates proposer unobserved"
         );
         assert_eq!(
-            cache.is_known(&sidecar_b),
+            cache.proposer_is_known(&sidecar_b),
             Ok(true),
             "observed block in slot 1 is indicated as true"
         );
@@ -322,30 +396,40 @@ mod tests {
 
         assert_eq!(cache.finalized_slot, 0, "finalized slot is zero");
         assert_eq!(cache.items.len(), 2, "two slots should be present");
+        let (cached_blob_indices, cached_block_roots) = cache
+            .items
+            .get(&ProposalKey::new(proposer_index_a, Slot::new(0)))
+            .expect("slot zero should be present");
         assert_eq!(
-            cache
-                .items
-                .get(&(proposer_index_a, Slot::new(0)))
-                .expect("slot zero should be present")
-                .len(),
+            cached_blob_indices.len(),
             1,
             "only one proposer should be present in slot 0"
         );
         assert_eq!(
-            cache
-                .items
-                .get(&(proposer_index_b, Slot::new(1)))
-                .expect("slot zero should be present")
-                .len(),
+            cached_block_roots.len(),
+            1,
+            "only one block root should be present in slot 0"
+        );
+        let (cached_blob_indices, cached_block_roots) = cache
+            .items
+            .get(&ProposalKey::new(proposer_index_b, Slot::new(1)))
+            .expect("slot zero should be present");
+        assert_eq!(
+            cached_blob_indices.len(),
             1,
             "only one proposer should be present in slot 1"
         );
+        assert_eq!(
+            cached_block_roots.len(),
+            1,
+            "only one block root should be present in slot 1"
+        );
 
         // Slot 0, index 1
         let sidecar_c = get_blob_sidecar(0, proposer_index_a, 1);
 
         assert_eq!(
-            cache.is_known(&sidecar_c),
+            cache.proposer_is_known(&sidecar_c),
             Ok(false),
             "no observation for new index"
         );
@@ -355,7 +439,7 @@ mod tests {
             "can observe new index, indicates sidecar unobserved for new index"
         );
         assert_eq!(
-            cache.is_known(&sidecar_c),
+            cache.proposer_is_known(&sidecar_c),
             Ok(true),
             "observed new sidecar is indicated as true"
         );
@@ -367,15 +451,57 @@ mod tests {
 
         assert_eq!(cache.finalized_slot, 0, "finalized slot is zero");
         assert_eq!(cache.items.len(), 2, "two slots should be present");
+        let (cached_blob_indices, cached_block_roots) = cache
+            .items
+            .get(&ProposalKey::new(proposer_index_a, Slot::new(0)))
+            .expect("slot zero should be present");
         assert_eq!(
-            cache
-                .items
-                .get(&(proposer_index_a, Slot::new(0)))
-                .expect("slot zero should be present")
-                .len(),
+            cached_blob_indices.len(),
             2,
             "two blob indices should be present in slot 0"
         );
+        // Changing the blob index doesn't change the block root, so only one unique signed
+        // header should be in the cache.
+        assert_eq!(
+            cached_block_roots.len(),
+            1,
+            "one block root should be present in slot 0"
+        );
+
+        // Create a sidecar sharing slot and proposer but with a different block root.
+        let mut sidecar_d: BlobSidecar<E> = BlobSidecar {
+            index: sidecar_c.index,
+            blob: sidecar_c.blob.clone(),
+            kzg_commitment: sidecar_c.kzg_commitment,
+            kzg_proof: sidecar_c.kzg_proof,
+            signed_block_header: sidecar_c.signed_block_header.clone(),
+            kzg_commitment_inclusion_proof: sidecar_c.kzg_commitment_inclusion_proof.clone(),
+        };
+        sidecar_d.signed_block_header.message.body_root = Hash256::repeat_byte(7);
+        assert_eq!(
+            cache.proposer_is_known(&sidecar_d),
+            Ok(true),
+            "there has been an observation for this proposer index"
+        );
+        assert_eq!(
+            cache.observe_sidecar(&sidecar_d),
+            Ok(true),
+            "indicates sidecar proposer was observed"
+        );
+        let (cached_blob_indices, cached_block_roots) = cache
+            .items
+            .get(&ProposalKey::new(proposer_index_a, Slot::new(0)))
+            .expect("slot zero should be present");
+        assert_eq!(
+            cached_blob_indices.len(),
+            2,
+            "two blob indices should be present in slot 0"
+        );
+        assert_eq!(
+            cached_block_roots.len(),
+            2,
+            "two block root should be present in slot 0"
+        );
 
         // Try adding an out of bounds index
         let invalid_index = E::max_blobs_per_block() as u64;
diff --git a/beacon_node/beacon_chain/src/observed_block_producers.rs b/beacon_node/beacon_chain/src/observed_block_producers.rs
index f76fc5379..096c8bff7 100644
--- a/beacon_node/beacon_chain/src/observed_block_producers.rs
+++ b/beacon_node/beacon_chain/src/observed_block_producers.rs
@@ -16,9 +16,15 @@ pub enum Error {
 }
 
 #[derive(Eq, Hash, PartialEq, Debug, Default)]
-struct ProposalKey {
-    slot: Slot,
-    proposer: u64,
+pub struct ProposalKey {
+    pub slot: Slot,
+    pub proposer: u64,
+}
+
+impl ProposalKey {
+    pub fn new(proposer: u64, slot: Slot) -> Self {
+        Self { slot, proposer }
+    }
 }
 
 /// Maintains a cache of observed `(block.slot, block.proposer)`.
diff --git a/beacon_node/http_api/src/publish_blocks.rs b/beacon_node/http_api/src/publish_blocks.rs
index 432d91b72..92d302f56 100644
--- a/beacon_node/http_api/src/publish_blocks.rs
+++ b/beacon_node/http_api/src/publish_blocks.rs
@@ -113,7 +113,10 @@ pub async fn publish_block<T: BeaconChainTypes, B: IntoGossipVerifiedBlockConten
     let (gossip_verified_block, gossip_verified_blobs) =
         match block_contents.into_gossip_verified_block(&chain) {
             Ok(b) => b,
-            Err(BlockContentsError::BlockError(BlockError::BlockIsAlreadyKnown)) => {
+            Err(BlockContentsError::BlockError(BlockError::BlockIsAlreadyKnown))
+            | Err(BlockContentsError::BlobError(
+                beacon_chain::blob_verification::GossipBlobError::RepeatBlob { .. },
+            )) => {
                 // Allow the status code for duplicate blocks to be overridden based on config.
                 return Ok(warp::reply::with_status(
                     warp::reply::json(&ErrorMessage {
@@ -185,6 +188,23 @@ pub async fn publish_block<T: BeaconChainTypes, B: IntoGossipVerifiedBlockConten
                     "slot" => block_clone.slot()
                 );
                 Err(BlockError::Slashable)
+            } else if chain_clone
+                .observed_blob_sidecars
+                .read()
+                .proposer_has_been_observed(
+                    block_clone.slot(),
+                    block_clone.message().proposer_index(),
+                    block_root,
+                )
+                .map_err(|e| BlockError::BeaconChainError(e.into()))?
+                .is_slashable()
+            {
+                warn!(
+                    log_clone,
+                    "Not publishing equivocating blob";
+                    "slot" => block_clone.slot()
+                );
+                Err(BlockError::Slashable)
             } else {
                 publish_block(
                     block_clone,