doppelganger is a ghostly double of a living person.
In the vernacular, the word "doppelganger: has come to refer to any double or look-alike of a person. The word is also used to describe the sensation of having glimpsed at oneself in peripheral vision, in a position where there is no chance that it could have been a reflection. They are generally regarded as harbingers of bad luck. In some traditions, a doppelganger seen by a person's friends or relatives portends illness or danger, while seeing one's own doppelganer is an omen of death.
I don't know about the harbingers of bad luck, but everywhere I turn at work I see some student that reminds me of someone I went to high school with or another student I taught while at Griffin. It's weird.... really weird