Heres my theory on de ja vu.
Theres a possibility that your dream is actually reality. Whereas the reality you are living in now is the dream. Basically reversed role.
If you went to a mall with friends in your dream 'the reality' then you do the exactly samething the next day when you 'wake up'. Same mall, same friends, bought exactly the samething. The things you did today is actually the dream. You see?
Even im getting a bit confused.