Replication should behave in an essentially synchronous manner.
What specifically is the use-case when you see these issues? Are you using transactions?
On another thread I've seen that when failing over to another server, my listeners re-connect correctly but log messages that were already handled prior to the fail over. This makes me wonder how could that be. If the replication is syhc - what other explanation can be that the ack on message handling was not "replicated" to the backup server (which would result in same message being handled twice)?
Perhaps the ack didn't make it to the server before the server crashed?
Got a reproducible test-case?