diff --git a/specs/altair/validator.md b/specs/altair/validator.md
index e6e00a674..3b3362b22 100644
--- a/specs/altair/validator.md
+++ b/specs/altair/validator.md
@@ -292,10 +292,10 @@ The validator broadcasts the assembled signature to the assigned subnet, the `sy
 The `subnet_id` is derived from the position in the sync committee such that the sync committee is divided into "subcommittees".
 `subnet_id` can be computed via `compute_subnets_for_sync_committee(state, validator_index)` where `state` is a `BeaconState` during the matching sync committee period.
 
-*Note*: This function returns multiple non-duplicated subnets if a given validator index is included multiple times in a given sync committee across multiple subcommittees.
+*Note*: This function returns multiple deduplicated subnets if a given validator index is included multiple times in a given sync committee across multiple subcommittees.
 
 ```python
-def compute_subnets_for_sync_committee(state: BeaconState, validator_index: ValidatorIndex) -> Sequence[uint64]:
+def compute_subnets_for_sync_committee(state: BeaconState, validator_index: ValidatorIndex) -> Set[uint64]:
     next_slot_epoch = compute_epoch_at_slot(Slot(state.slot + 1))
     if compute_sync_committee_period(get_current_epoch(state)) == compute_sync_committee_period(next_slot_epoch):
         sync_committee = state.current_sync_committee
diff --git a/tests/core/pyspec/eth2spec/test/altair/unittests/validator/test_validator.py b/tests/core/pyspec/eth2spec/test/altair/unittests/validator/test_validator.py
index cefaaf694..18fb730d7 100644
--- a/tests/core/pyspec/eth2spec/test/altair/unittests/validator/test_validator.py
+++ b/tests/core/pyspec/eth2spec/test/altair/unittests/validator/test_validator.py
@@ -149,9 +149,10 @@ def _subnet_for_sync_committee_index(spec, i):
 
 
 def _get_expected_subnets_by_pubkey(sync_committee_members):
-    expected_subnets_by_pubkey = defaultdict(list)
+    # Build deduplicated set for each pubkey
+    expected_subnets_by_pubkey = defaultdict(set)
     for (subnet, pubkey) in sync_committee_members:
-        expected_subnets_by_pubkey[pubkey].append(subnet)
+        expected_subnets_by_pubkey[pubkey].add(subnet)
     return expected_subnets_by_pubkey