diff --git a/test/functional/test_framework/mempool_util.py b/test/functional/test_framework/mempool_util.py
index e04ae914ccfd9..0e9c821e2ead9 100644
--- a/test/functional/test_framework/mempool_util.py
+++ b/test/functional/test_framework/mempool_util.py
@@ -30,6 +30,7 @@ def assert_mempool_contents(test_framework, node, expected=None, sync=True):
         test_framework.sync_mempools()
     if not expected:
         expected = []
+    assert_equal(len(expected), len(set(expected)))
     mempool = node.getrawmempool(verbose=False)
     assert_equal(len(mempool), len(expected))
     for tx in expected: