Dumb (?) subclass question

Hi all,

I just spent four hours searching for a bug when I found a situation in 
which a to-many relationship always contained duplicate elements.
I always was under the impression that the array I get will contain 
distinct rows and never checked for duplicates.

Let's say there are two tables: HEAD and ITEM with a 1:n relationship.

Whenever I say (HEAD)h.items(), I get four entries in the NSArray when 
in the database there are only three.

Could this be somehow due to the fact that i sublassed ITEM?

If so, how can I prevent this behaviour?

Greetings,

- Thomas