In addition to what other people have posted, you could also get deja vu simply because you have in fact done something like what you are currently doing before, even if you don't remember precisely when. I mean, if you walk down a street and see a church, and get a feeling of deja vu, well, you almost certainly have walked down streets and seen churches before. The brain makes the connection but fails to pull up a specific example, so you get that feeling.