Ensure that we don't have duplicate hosts in the pdu destinations list

synapse/federation/replication.py

@@ -509,10 +509,10 @@ class _TransactionQueue(object):
         # a transaction in progress. If we do, stick it in the pending_pdus
         # table and we'll get back to it later.
 
-        destinations = [
+        destinations = set([
             d for d in pdu.destinations
             if d != self.server_name
-        ]
+        ])
 
         logger.debug("Sending to: %s", str(destinations))