A doppelganger is a loose term that is used to describe any sort of lookalike. In folklore, it is a ghost with no shadow, and seeing it is a sign of bad luck. For instance, seeing a doppelganger of a relative means that the relative shall soon die.
Today, doppelgangers are seen as an evil twin, meaning it looks similar to a deceased or a living person. In German, the word means "double-goer." It can appear in the flesh and not just a ghostly figure. The haunting activity is considered a warning for danger, illness or even death.
There are doppelgangers in many works of fiction as well, such as in Edgar Allan Poe's "William Wilson" and Stephen King's "The Dark Half." Apparitions that appear in these pieces of work always bring a sign of bad luck.