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.
Others are full fledged people you encounter out in the world. People have chased their doppelgangers, confronted them, been confronted by them, and run away from them. Sometimes your doppelganger is encountered by someone else. Let's say someone meets "you" in Paris, while you yourself are on vacation in Melbourne. Classic doppelganger story. The "you" in Paris looks, talks, and acts exactly like you - but it couldn't possibly be you, because you are not there.
Doppelgangers are typically thought to be ill omens and harbingers of bad luck. No surprise there. A doctor recently worried that she might die soon because someone saw her doppelganger (warning: bad autoplay video) and believes it foretold her death. The doctor was in Germany at the same time that a friend encountered "her" in Singapore.
Interestingly, the doctor was meditating at the time that her doppelganger was spotted in Singapore. This feeds into the theory of bilocation: that doppelgangers are simply a manifestation of your spirit's ability to unhook itself from your body and travel freely. A sort of high fidelity astral projection, if you will.
Abraham Lincoln, Queen Elizabeth I, Percy Shelley, and Guy de Maupassant all had doppelganger experiences not long before they died. Lincoln saw his double in a mirror with a death mask. Queen Elizabeth died shortly after seeing her double laid out on her own bed. Shelley's friend saw him pass by a window twice when he was elsewhere. John Donne saw his wife's doppelganger the same night that his daughter was stillborn. De Maupassant reported being plagued by his doppelganger in the days before his death, and claimed that at one point his double came into his room, sat down on a chair, and started dictating what de Maupassant was writing at the time.
On the other hand, not all doppelgangers spell doom. Goethe saw his own doppelganger coming towards him on a road, wearing unfamiliar clothing. Eight years later he happened to be riding down that road, and realized he was wearing the outfit he had seen himself in before.