Comments

src/github.com/matrix-org/dendrite/federationsender/consumers/roomserver.go

@@ -137,6 +137,8 @@ func (s *OutputRoomEvent) processMessage(ore api.OutputNewRoomEvent) error {
 	if oldJoinedHosts == nil {
 		// This means that there is nothing to update as this is a duplicate
 		// message.
+		// This can happen if dendrite crashed between reading the message and
+		// persisting the stream position.
 		return nil
 	}
 

src/github.com/matrix-org/dendrite/federationsender/storage/storage.go

@@ -84,7 +84,9 @@ func (d *Database) UpdateRoom(
 		}
 
 		if lastSentEventID == newEventID {
-			// We've handled this messages before, so lets just ignore it
+			// We've handled this message before, so let's just ignore it.
+			// We can only get a duplicate for the last message we processed,
+			// so its enough just to compare the newEventID with lastSentEventID
 			return nil
 		}