Only count aggregations from distinct senders

changelog.d/5203.feature

@@ -0,0 +1 @@
+Add experimental support for relations (aka reactions and edits).

synapse/storage/relations.py

@@ -280,7 +280,7 @@ def get_aggregation_groups_for_event(
             having_clause = ""
 
         sql = """
-            SELECT type, aggregation_key, COUNT(*), MAX(stream_ordering)
+            SELECT type, aggregation_key, COUNT(DISTINCT sender), MAX(stream_ordering)
             FROM event_relations
             INNER JOIN events USING (event_id)
             WHERE {where_clause}