In our setup we have two brokers clustered together with redistribution enabled. We have a multicast address with about a dozen subscriptions bound to it. What we are observing is that if a consumer of one of those subscriptions goes offline, messages will begin to build up in the queue (as expected). When the consumer comes online, whatever broker it attaches to first will have messages redistributed from the other broker in the cluster. When this redistribution happens the other multicast queues receive some of those redistributed messages (although not all of them), even though they had already received them previously.