diff --git a/swh/indexer/storage/in_memory.py b/swh/indexer/storage/in_memory.py
--- a/swh/indexer/storage/in_memory.py
+++ b/swh/indexer/storage/in_memory.py
@@ -166,8 +166,7 @@
             if conflict_update:
                 item_ctags = []
             else:
-                # TODO: this merges old ctags with new ctags. This is
-                # pointless, new ctags should replace the old ones.
+                # merge old ctags with new ctags
                 existing = list(self._content_ctags.get([item['id']]))
                 item_ctags = [
                     {
diff --git a/swh/indexer/tests/test_ctags.py b/swh/indexer/tests/test_ctags.py
--- a/swh/indexer/tests/test_ctags.py
+++ b/swh/indexer/tests/test_ctags.py
@@ -107,8 +107,13 @@
     - Unknown sha1 in the input list are not indexed
 
     """
+
+    def get_indexer_results(self, ids):
+        yield from self.idx_storage.content_ctags_get(ids)
+
     def setUp(self):
         self.indexer = CtagsIndexerTest()
+        self.idx_storage = self.indexer.idx_storage
 
         # Prepare test input
         self.id0 = '01c9379dfc33803963d07c1ccc748d3fe4c96bb5'
diff --git a/swh/indexer/tests/test_utils.py b/swh/indexer/tests/test_utils.py
--- a/swh/indexer/tests/test_utils.py
+++ b/swh/indexer/tests/test_utils.py
@@ -659,7 +659,13 @@
 
 
 class CommonContentIndexerTest:
-    def assert_results_ok(self, actual_results, expected_results=None):
+    def get_indexer_results(self, ids):
+        """Override this for indexers that don't have a mock storage."""
+        return self.indexer.idx_storage.state
+
+    def assert_results_ok(self, sha1s, expected_results=None):
+        actual_results = self.get_indexer_results(sha1s)
+
         if expected_results is None:
             expected_results = self.expected_results
 
@@ -678,15 +684,12 @@
         # when
         self.indexer.run(sha1s, policy_update='update-dups')
 
-        actual_results = self.indexer.idx_storage.state
-        self.assertTrue(self.indexer.idx_storage.conflict_update)
-        self.assert_results_ok(actual_results)
+        self.assert_results_ok(sha1s)
 
         # 2nd pass
         self.indexer.run(sha1s, policy_update='ignore-dups')
 
-        self.assertFalse(self.indexer.idx_storage.conflict_update)
-        self.assert_results_ok(actual_results)
+        self.assert_results_ok(sha1s)
 
     def test_index_one_unknown_sha1(self):
         """Unknown sha1 are not indexed"""
@@ -696,14 +699,13 @@
 
         # when
         self.indexer.run(sha1s, policy_update='update-dups')
-        actual_results = self.indexer.idx_storage.state
 
         # then
         expected_results = {
             k: v for k, v in self.expected_results.items() if k in sha1s
         }
 
-        self.assert_results_ok(actual_results, expected_results)
+        self.assert_results_ok(sha1s, expected_results)
 
 
 class CommonContentIndexerRangeTest: