Don't allow accepting an introduction of two identities belonging to the same contact
The first user has identity H, the second user S1 and S2. H and S1 are contacts; H and S2 are contacts.
The first user introduces S1 to S2. For some inexplicable reason, the second user accepts the introduction to herself, so she ends up with her own identities on her contact list: S1 as a contact of S2, and S2 as a contact of S1.
So far this isn't necessarily a bug, although it's probably a bad idea.
The bug is that when the second user touches S1 or S2 on the contact list, the introduction with S1 and S2 is always shown.