<< I heard one theory that said that the mind is processing the current even so rapidly (storing it to memory) that one perceives it both as the present and as an existing memory >>
Something like that, or maybe old memories get merged and mixed, and when the mind processes what it is receiving, it compares various sections with those old memories, giving the illusion or repetition.
I get deja vu sometimes of having had deja vu of something before. It gets kind of weird and almost annoying sometimes.
"I swear I did this before, and last time I did this, I was having deja vu." - Like that.