The phenomenon Déjà vu is known to most people, as having experienced something before when it is happening; See here for the general information on it; I am here to explain to you what the occult and real reason behind it is.

Déjà vu is simply put, a minor ability possessed by people with clairvoyant traits or inclinations, to briefly see into the future. I have had this many, many times when I react like “I know this”- I have dreamt for a couple of seconds, numerous times, that e.g. I worked somewhere and it is only years after, when the moment arrives that I go like “I know this”. This is something that a lot of people have, and normal. It is different than a dream, where you can remember things, depending on a number of factors.
Déjà vu is different; It’s like watching a movie for 3 to 7 seconds of a really bad quality, but you are actually there in that event, feeling & hearing. Unfortunately, most are not able to make sense of it at that time and dismiss it. It is only then, when the moment itself arrives, that one feels like it has occurred before. I have seen people whose names I didn’t know then, but when I meet them I felt like I knew them or that I’ve seen them before, you name it..

See here as well for more information on dream-related matters