Sub step 2 in 'for each MediaStreamTrack' says

"If track has ended or if there is already a MediaStreamTrack contained within stream that has the same underlying source as track, then abort these steps and continue with the next track (if any)."

Denying the possibility to have more than one track with the same source in a MediaStream made sense back in the days when source and track where more or less the same thing.

With the development where several tracks can be associated with the same source, and ask for different operation modes this does not make sense any more. Rather, it is limiting - having several tracks representing the same source in one MediaStream can enable e.g. simulcast (by applying different constraints).

I think we should remove "or if there is already a MediaStreamTrack contained within stream that has the same underlying source as track," from sub step 2.