I remember having strong deja vu when I was a kid, it would freak me out so much that Id just walk away from situations. As Ive gotten older I think I still experience them but I attribute it to nothing more than my mind playing tricks on me and quickly disregard it as fast as it happens. If I had to guess what it actually is, I would say its the mind processing things into memory as fast as you're experiencing them, which simultaneously tricks your brain into believing you're living out a situation you've already experienced. Even though that's not the case. I remember and get the strong feeling I can predict what someone is going to say, but when I've realized it's happening and tried to actually say what they'll say before they say it, I cant, even though it feels like I should be able to.