Event time I save my object after adding children it bombs.

For instance.. Class a has a bag of items ( id's 12, 13, and 14 )

I have already saved to the collection so they exist in a many to many mapping table.

My question is, now do I tell it to skip re-adding pre-existing entries.

When I save it tries to save id's 12, 13, and 14 even though already exist. And ideas?

{"a different object with the same identifier value was already associated with the session: 12, of entity: Class Name Here"}
<class name="DocumentSavingsEvent, DomainObjects" table="DCSEvent" lazy="false">

<id name="Id" column="EventId" unsaved-value="-1">
<generator class="native" />
</id>

<bag name="ActivityListItems" table="DCSEventActivityItemTypeMapping" lazy="false" cascade="save-update" inverse="false">
<key column="EventId" />
<many-to-many class="DocumentSavingsCostActivityItem, DomainObjects" column="ActivityListId" />
</bag>
</class>

Thanks,
Nick