The official explanation that was totally not in the movie at all is essentially Palpatine created clones as a precaution as he sensed Vader would turn to the light side, and used his powers to transfer his will into a clone while he fell to his "death." One of these clones ended up as a failure so he transferred to another clone, the one you see in the movie. The failed clone is Rey's father.
I dunno. Just seems like bullshit to try to explain away a plothole they didn't realize was there.