Scientifically, deja vu is a 'hiccup' in our memory store. There are studies on it. One of the most recent is false memory, the brain checks the memory store to link the situation to anything that may have similarities to past events.
Another is, the sensory experience and the processing of the event get slightly out of sync, thus what we subconsciously experience via our senses feels as though we have experienced it before as the experience feeds into the memory.

Spiritually, we may put it down to precognition, a past life experience or a loop in the time/space continuum.