I am getting the following exception upon commit:

Caused by: org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session: [org.freebxml.omar.server.event.SubscriptionStatus#urn:test:subscription:periodic1]
at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:168)
at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:121)
at org.hibernate.ejb.event.EJB3PersistEventListener.saveWithGeneratedId(EJB3PersistEventListener.java:49)
at org.hibernate.event.def.DefaultPersistEventListener.entityIsTransient(DefaultPersistEventListener.java:131)
at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:87)
at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:38)
at org.hibernate.impl.SessionImpl.firePersist(SessionImpl.java:618)
at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:592)
at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:596)
at org.hibernate.ejb.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:220)

The entity in question was fetched by EntityManager #1 and cached. In a seperate thread it was merged into EntityManager #2 and modified and committed when I got the excpetion.

What I do not understand is why I am getting this if I called entityManager2.merge(entity) on the said entity.

Am I misunderstanding how merge is supposed to work?

TIA for your help.