As stated in the Sci Fi Series, Fringe
(#3 in the list of Fringe), deja vu occurs when you have done something before, hence feeling you've done the same as what you're doing in your reality. In truth, you've done it, but in an alternate reality.
Tom: Woah, Dude. I totally feel like I've seen that chick before.
David: Yeah, man, I know.
Tom: Well she wouldn't ask me out even if it was in an alternate Reality.
David: Yeah, man. Totally.
Tom: Just a rush of Deja Vu.