Fix sending a sync message to the same device twice.

[boxdot]

This happens when the session store contains overlapping sessions for
the uuid and the e164 phone number of the same account. A device
which has both session receives and shows a sync message twice.

[rubdos]

You should never have overlapping sessions. The remote end cannot distinguish them, and this leads to undecryptable messages.

You should instead have logic like this merge_and_fetch_recipient, which decides when a phone number is moved or tied to a UUID. Better yet, Android's RecipientDatabase::getAndPossiblyMerge. An e164 and UUID that are "together" should never have separate sessions.

[rubdos]

@boxdot , are you okay with closing this? Maybe we should document these requirements somehow, but it'd quite a challenge to document how to "correctly" implement the whole of Signal.

Maybe another approach here could be to give an error when overlapping sessions are retrieved, à la "This is a bug in the application and should be reported. The session with XYZ is now probably corrupt.".

[boxdot]

I still don't have a working solution in my client. But I will close it.

[rubdos]

I still don't have a working solution in my client. But I will close it.

If you want, you can open an issue about session/uuid/e164 management and merging; it should be relatively easy to port the (currently incomplete) session merging logic from Whisperfish into here.