I get it pretty often, just certain situations feeling really familiar, but more like I've dreamed about them than ever experienced them before.
I've heard before that it's because the signal from one eye sometimes reaches the brain faster than the other, meaning you literally have seen and processed the same thing before, and also that areas of the brain sometimes process the same information at marginally different speeds, but I don't know if there's any truth in either. Considering how little we know about the brain, I doubt anyone really knows. :P