Comment 2 for bug 1274204

I don't think the DeeModel itself enforces uniqueness on the message ID (it's not like a database where you'd index the ID), but in the friends backend there's some code that builds dicts mapping IDs to rows, so probably if there were two IDs you might get some goofy issues (things like deletion might break, as it would only delete one copy and not the other from the model).