-ImaginaryInsider
Member
This could be complete bullshit, but the story I recall was that the "other" was a completely new character that wasn't intended to show up in the original trilogy, and while developing RotJ he condensed a lot of stuff down into that movie (like the final showdown with Palpatine). In the process the "other" got merged into Leia because it didn't make sense to bring in such a significant new character at the last second, and it fit in fairly well with the narrative arc he wanted (awkward incest-y moments aside).
It wouldn't surprise me, there were tentative plans for Obi-Wan and Yoda to rematerialize back to life and help Luke at one point in ROTJ.
Oh and there were double Death Stars as well...