diff --git a/eth2/state_processing/src/per_block_processing.rs b/eth2/state_processing/src/per_block_processing.rs
index 8634bb0e129..a4f2be87ef4 100644
--- a/eth2/state_processing/src/per_block_processing.rs
+++ b/eth2/state_processing/src/per_block_processing.rs
@@ -283,26 +283,25 @@ pub fn process_proposer_slashings<T: EthSpec>(
     verify_signatures: VerifySignatures,
     spec: &ChainSpec,
 ) -> Result<(), BlockProcessingError> {
-    // Verify proposer slashings in parallel.
+    // Verify and apply proposer slashings in series.
+    // We have to verify in series because an invalid block may contain multiple slashings
+    // for the same validator, and we need to correctly detect and reject that.
     proposer_slashings
-        .par_iter()
+        .into_iter()
         .enumerate()
         .try_for_each(|(i, proposer_slashing)| {
             verify_proposer_slashing(proposer_slashing, &state, verify_signatures, spec)
-                .map_err(|e| e.into_with_index(i))
-        })?;
+                .map_err(|e| e.into_with_index(i))?;
 
-    // Update the state.
-    for proposer_slashing in proposer_slashings {
-        slash_validator(
-            state,
-            proposer_slashing.signed_header_1.message.proposer_index as usize,
-            None,
-            spec,
-        )?;
-    }
+            slash_validator(
+                state,
+                proposer_slashing.signed_header_1.message.proposer_index as usize,
+                None,
+                spec,
+            )?;
 
-    Ok(())
+            Ok(())
+        })
 }
 
 /// Validates each `AttesterSlashing` and updates the state, short-circuiting on an invalid object.
diff --git a/eth2/state_processing/src/per_block_processing/tests.rs b/eth2/state_processing/src/per_block_processing/tests.rs
index f942cc29f56..22f30bd265a 100644
--- a/eth2/state_processing/src/per_block_processing/tests.rs
+++ b/eth2/state_processing/src/per_block_processing/tests.rs
@@ -1029,6 +1029,36 @@ fn invalid_proposer_slashing_not_slashable() {
     );
 }
 
+#[test]
+fn invalid_proposer_slashing_duplicate_slashing() {
+    let spec = MainnetEthSpec::default_spec();
+    let builder = get_builder(&spec, SLOT_OFFSET, VALIDATOR_COUNT);
+    let test_task = ProposerSlashingTestTask::Valid;
+    let (mut block, mut state) =
+        builder.build_with_proposer_slashing(test_task, 1, None, None, &spec);
+
+    let slashing = block.message.body.proposer_slashings[0].clone();
+    let slashed_proposer = slashing.signed_header_1.message.proposer_index;
+    block.message.body.proposer_slashings.push(slashing);
+
+    let result = per_block_processing(
+        &mut state,
+        &block,
+        None,
+        BlockSignatureStrategy::NoVerification,
+        &spec,
+    );
+
+    // Expecting ProposerNotSlashable because we've already slashed the validator
+    assert_eq!(
+        result,
+        Err(BlockProcessingError::ProposerSlashingInvalid {
+            index: 1,
+            reason: ProposerSlashingInvalid::ProposerNotSlashable(slashed_proposer)
+        })
+    );
+}
+
 #[test]
 fn invalid_bad_proposal_1_signature() {
     let spec = MainnetEthSpec::default_spec();