diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index 7ce93eb6f..8234c9324 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -628,8 +628,8 @@ def is_valid_indexed_attestation(state: BeaconState, indexed_attestation: Indexe
     # Verify max number of indices
     if not len(indices) <= MAX_VALIDATORS_PER_COMMITTEE:
         return False
-    # Verify indices are sorted
-    if not indices == sorted(indices):
+    # Verify indices are sorted and unique
+    if not indices == sorted(set(indices)):
         return False
     # Verify aggregate signature
     if not bls_verify(
diff --git a/test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_attester_slashing.py b/test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_attester_slashing.py
index 85e807ec0..3fc2f8a38 100644
--- a/test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_attester_slashing.py
+++ b/test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_attester_slashing.py
@@ -252,6 +252,66 @@ def test_att2_bad_replaced_index(spec, state):
     yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
 
 
+@with_all_phases
+@spec_state_test
+@always_bls
+def test_att1_duplicate_index_normal_signed(spec, state):
+    attester_slashing = get_valid_attester_slashing(spec, state, signed_1=False, signed_2=True)
+
+    indices = attester_slashing.attestation_1.attesting_indices
+    indices.pop(1)  # remove an index, make room for the additional duplicate index.
+    indices.append(indices[0])  # add one of the indices a second time
+    attester_slashing.attestation_1.attesting_indices = sorted(indices)
+    sign_indexed_attestation(spec, state, attester_slashing.attestation_1)
+    # it will just appear normal, unless the double index is spotted
+    yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
+
+
+@with_all_phases
+@spec_state_test
+@always_bls
+def test_att2_duplicate_index_normal_signed(spec, state):
+    attester_slashing = get_valid_attester_slashing(spec, state, signed_1=True, signed_2=False)
+
+    indices = attester_slashing.attestation_2.attesting_indices
+    indices.pop(2)  # remove an index, make room for the additional duplicate index.
+    indices.append(indices[1])  # add one of the indices a second time
+    attester_slashing.attestation_2.attesting_indices = sorted(indices)
+    sign_indexed_attestation(spec, state, attester_slashing.attestation_2)
+    # it will just appear normal, unless the double index is spotted
+    yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
+
+
+@with_all_phases
+@spec_state_test
+@always_bls
+def test_att1_duplicate_index_double_signed(spec, state):
+    attester_slashing = get_valid_attester_slashing(spec, state, signed_1=False, signed_2=True)
+
+    indices = attester_slashing.attestation_1.attesting_indices
+    indices.pop(1)  # remove an index, make room for the additional duplicate index.
+    indices.append(indices[2])  # add one of the indices a second time
+    attester_slashing.attestation_1.attesting_indices = sorted(indices)
+    sign_indexed_attestation(spec, state, attester_slashing.attestation_1)  # will have one attester signing it double
+
+    yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
+
+
+@with_all_phases
+@spec_state_test
+@always_bls
+def test_att2_duplicate_index_double_signed(spec, state):
+    attester_slashing = get_valid_attester_slashing(spec, state, signed_1=True, signed_2=False)
+
+    indices = attester_slashing.attestation_2.attesting_indices
+    indices.pop(1)  # remove an index, make room for the additional duplicate index.
+    indices.append(indices[2])  # add one of the indices a second time
+    attester_slashing.attestation_2.attesting_indices = sorted(indices)
+    sign_indexed_attestation(spec, state, attester_slashing.attestation_2)  # will have one attester signing it double
+
+    yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
+
+
 @with_all_phases
 @spec_state_test
 def test_unsorted_att_1(spec, state):