Could they be standing behind you right now?
I have been fascinated by stories of doppelgangers ever since I was a little kid. A doppelganger is your exact double. From there, it all varies.
Many doppelgangers are glimpsed in the peripheral vision, only briefly, but under circumstances where it could not possibly be a reflection. You catch a glimpse of yourself out of the corner of your eye, but there is no mirror there.