diff --git a/ann_benchmarks/runner.py b/ann_benchmarks/runner.py
index 4916e13e..81428114 100644
--- a/ann_benchmarks/runner.py
+++ b/ann_benchmarks/runner.py
@@ -66,6 +66,10 @@ def single_query(v: numpy.array) -> Tuple[float, List[Tuple[int, float]]]:
                 start = time.time()
                 candidates = algo.query(v, count)
                 total = time.time() - start
+
+            # make sure all returned indices are unique
+            assert len(candidates) == len(set(candidates)), "Implementation returned duplicated candidates"
+
             candidates = [
                 (int(idx), float(metrics[distance].distance(v, X_train[idx]))) for idx in candidates  # noqa
             ]
@@ -105,6 +109,11 @@ def batch_query(X: numpy.array) -> List[Tuple[float, List[Tuple[int, float]]]]:
                 batch_latencies = algo.get_batch_latencies()
             else:
                 batch_latencies = [total / float(len(X))] * len(X)
+
+            # make sure all returned indices are unique
+            for res in results:
+                assert len(res) == len(set(res)), "Implementation returned duplicated candidates"
+
             candidates = [
                 [(int(idx), float(metrics[distance].distance(v, X_train[idx]))) for idx in single_results]  # noqa
                 for v, single_results in zip(X, results)