If I understand what you did correctly, JBoss worked as expected. You established a relationship between, say,
A1 and Z1. Then you tried to establish a relationship
between A2 and Z1. But Z1 can't be related to both
A1 and A2 since the relation is supposed to be 1-to-1.
It looks like many-to-one unidirectional - at least that's how I can see it now.
Thanks for explanation.