duplicate delivery, already consumed messages are reconsumed after server restart

I am running an embedded activemq broker inside jboss server, I send 1000 messages with ~10K size each to a queue, these messages are consumed by MDBs. After all the messages are consumed, I check the queue via hermes, it also shows no message in the queue. Everything works perfect up to this point. I observe the problem after I stop the jboss server. I connect to derby database via networkserver, I still see some messages in activemq_msgs table. I shutdown derby networkserver and start jboss again, I see from the log that some of the messages which were consumed previously, are being consumed again. If I start the server without deploying the MDB and check the queue via hermes, I see some but not all the messages which were consumed previously still in the queue, before restart hermes was showing no messages.