org.jboss.mq.server.MessageReference does override equals() (and even compareTo()), but not hashCode().
This is a potential issue, when adding MessageReferences to hashCode-based Collections such as Hashmap or HashSet.

from java.lang.Object:
If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.

Will append a patch.

Came across this when searching for the root of a symptom, where messages were processed and removed from the MessageCache, but still were persistent in database - don't know yet if it's related.