I, like many others online, am thinking Dark Rey is a clone.
My theory is Rey and her clones are the result of Palpatine’s experiments, which began long ago. Getting cloning tech access, and perfecting it to use on Force wielders (whether for an army or vessels for Palpatine’s spirit, tbd) were the Clone Wars’ other purpose beyond exterminating the Jedi - tying everything together from the beginning.
Also would explain why Rey sees reflections of herself in the cave on Ahch-To when she asks to see her parents, as others have mentioned.
I really think this might be it. It gives an even larger scale to Palpatine’s schemes from TPM through the Clone Wars and beyond.