Fixed issue with duplicate failed messages not being removed from the queue

A duplicate of a previous message that has eventually failed (due to the execution or exception limit been exceeded) was not ACKed thus preventing it being removed from the queue and repeatedly reprocessed until the time out limit was reached.  This also caused a sleep of 1 second until the next message (from either queue) was processed.

In RabbitMQ v >=2.7.0 this was a bigger problem as the message would stay at the front of the queue thus blocking other messages, and therefore the 1 second sleep would be continuously repeated until the time out limit was reached (default 10 minutes) and the message finally removed.