Brian Earl Spilner said:
If it was "real", it would be even more upsetting that it was spoiled.
No chance.
I am thinking Dark-Rey is Rey.0 who the emperor cloned from Luke's own hand ... the Emperor's "Hand" who later cloned herself and infiltrated her clone into Luke's Jedi Academy, or who the emperor cloned for his own reasons.
And Rey.X is who we know as the character Rey... it would explain:
1. Rey's innate ability in the force
2. Rey's vision on Ahch-to
3. Rey's immediate draw to the Darkside Pit on Ahch-to during her lesson #1 from Luke
4. Rey's fighting style (at least to me) reminding me of Sidious' big overhead thrusts
So Rey is the Clone of a Clone of Luke (her parents are nobody). And Kylo knows both Rey.0 from before (perhaps part of the Knights of Ren?) and that is why he wants to know who Rey.X is...
Anyhoo... Dark Rey.0 looks like an OLDER (and thinner and sexier) version of Rey.X. Except she may be as bald as Sinead O'Connor under there....