Doppelgänger
‘däpəlˌɡaNGər/
noun

Is a look-alikeor double of a living person, sometimes portrayed as a ghostly or paranormal-phenomenon and usually seen as a harbinger of bad luck

The history of Doppelgängers was first discovered in German Mythology, they were usually described as evil in nature and would normally harm their look alike or cause them to harm themselves by putting malicious thoughts in their minds.
Doppelgängers can appear to us in many different ways, we can see them out of the corner of our eyes, noticing them in the distance, or standing behind us while we look at our reflection.

There are several documented stories about someone seeing their double, and sadly, it has ended in death.