Some times that shyt is SO vivid you feel you just lived the same thing twice
BUT I read a theory that basically goes like this: we have two types memories, short and long term. What happens is when we see a particular situation walking down the street, it should go in th short-term memory, the one that is "erased" if the info isn't worth keeping. The thing is, it seems that sometimes our brain fukks up, and instead of putting that particuler scene in the short-term memory bank, it puts it in the long-term memory bank making us think "shyt, I KNOW I've seen this before"
That may explain why most "déja vus" are really memories that aren't important, which are typically short-term memories that get erased. BUt for some reason, sometimes they end up in the long-term memory bank.
Now mind you this is only a theory
first thing I thought when I read it was like "Yeah right I KNOW FOR A FACT I saw that scene before
" so take it as you wish...