diff --git a/synapse/storage/schema/delta/16/remove_duplicates.sql b/synapse/storage/schema/delta/16/remove_duplicates.sql
new file mode 100644
index 0000000000..65c97b5e2f
--- /dev/null
+++ b/synapse/storage/schema/delta/16/remove_duplicates.sql
@@ -0,0 +1,9 @@
+
+
+DELETE FROM event_to_state_groups WHERE state_group not in (
+    SELECT MAX(state_group) FROM event_to_state_groups GROUP BY event_id
+);
+
+DELETE FROM event_to_state_groups WHERE rowid not in (
+    SELECT MIN(rowid) FROM event_to_state_groups GROUP BY event_id
+);