Description of problem:
A singleton MDB delivers duplicate messages when start-delivery() is invoked twice while the message being delivered

Steps to Reproduce:
1. Create a producer, which keeps sending messages for every second
2. Deploy a singleton consumer MDB
3. Invoke start-delivery() method on the MDB while the messages being delivered

Actual results:
MDB delivers duplicate messages

Expected results:
MDB needs to ignore start-delivery() call if the MDB has already been started.

Additional info: