diff --git a/changelog.d/9210.bugfix b/changelog.d/9210.bugfix
new file mode 100644
index 0000000000..f9e0765570
--- /dev/null
+++ b/changelog.d/9210.bugfix
@@ -0,0 +1 @@
+Fix chain cover update to handle events with duplicate auth events. Introduced in v1.26.0rc1.
diff --git a/synapse/util/iterutils.py b/synapse/util/iterutils.py
index 6ef2b008a4..8d2411513f 100644
--- a/synapse/util/iterutils.py
+++ b/synapse/util/iterutils.py
@@ -78,7 +78,7 @@ def sorted_topologically(
if node not in degree_map:
continue
- for edge in edges:
+ for edge in set(edges):
if edge in degree_map:
degree_map[node] += 1
diff --git a/tests/util/test_itertools.py b/tests/util/test_itertools.py
index 522c8061f9..1ef0af8e8f 100644
--- a/tests/util/test_itertools.py
+++ b/tests/util/test_itertools.py
@@ -92,3 +92,15 @@ class SortTopologically(TestCase):
# Valid orderings are `[1, 3, 2, 4]` or `[1, 2, 3, 4]`, but we should
# always get the same one.
self.assertEqual(list(sorted_topologically([4, 3, 2, 1], graph)), [1, 2, 3, 4])
+
+ def test_duplicates(self):
+ "Test that a graph with duplicate edges work"
+ graph = {1: [], 2: [1, 1], 3: [2, 2], 4: [3]} # type: Dict[int, List[int]]
+
+ self.assertEqual(list(sorted_topologically([4, 3, 2, 1], graph)), [1, 2, 3, 4])
+
+ def test_multiple_paths(self):
+ "Test that a graph with multiple paths between two nodes work"
+ graph = {1: [], 2: [1], 3: [2], 4: [3, 2, 1]} # type: Dict[int, List[int]]
+
+ self.assertEqual(list(sorted_topologically([4, 3, 2, 1], graph)), [1, 2, 3, 4])
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