Correctly handle sessions that are in multiple tracks

I’m not super sure that allowing duplicates in the list makes sense, but also if you filter by a particular track I want to make sure they’re all in there. So if you do the right filters, you’ll see multiples of the same thing for now. Fiddling with the upstream stuff would take more work.

So I: • uniqued them properly by a identifier + trackName • made the track assignment in the view model deterministic instead of session.tracks.first.