Throughout history, mirrors have been used for divination and such. They are also traditionally associated with fear and evil in several cultures. Others believe that you can see your soul when you look into the mirror, so that could be the case with you.
As far as the deja vu goes, it could signal something about a past life experience. It's possible that you saw your soul in the mirror and that something similar happened in a previous incarnation, or that you saw a particular past life in the mirror, which could potentially explain why it felt like you but wasn't quite you. Sorry if the explanation is a bit muddled; it's weird to explain. Just message me if you want me to explain anything.