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Your brain gives off chemicals when you remember something. I've had like mild de ja vu and I think that's just from being real familiar with an actual event in the past, but de ja vu is when those chemicals are fired when you actually don't remember something happening to you for a "second time." If you don't believe this, the next time you or a friend has it, have them or yourself predict what will happen. If something happens and the reaction is "i knew you were going to do/say that!" It's the chemicals.

That is a premonition, not déjà vu.

Déjà vu it the feeling that you've already experienced what you're experiencing now. A premonition is the feeling that you're going to experience something at some time in the future.