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A doppelgänger (pronunciation (help·info)) or fetch is the ghostly double of a living person, a sinister form of bilocation.

In the vernacular, "Doppelgänger" has come to refer (as in German) to any double or look-alike of a person—most commonly an "evil twin". The literal translation of the German word is "doublewalker", meaning someone who is acting (e.g. walking) the same way as another person. The word is also used to describe the sensation of having glimpsed 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 doppelgänger seen by a person's friends or relatives portends illness or danger, while seeing one's own doppelgänger is an omen of death. In Norse mythology, a vardøger is a ghostly double who precedes a living person and is seen performing their actions in advance.

The doppelgängers of folklore cast no shadow, and have no reflection in a mirror or in water. They are supposed to provide advice to the person they shadow, but this advice can be misleading or malicious. They can also, in rare instances, plant ideas in their victim's mind or appear before friends and relatives, causing confusion. In many cases once someone has viewed his own doppelgänger he is doomed to be haunted by images of his ghostly counterpart.

Other folklore says that when a person's doppelgänger is seen, the person him/herself will die shortly. It is considered unwise to try to communicate with a doppelgänger.

In some beliefs, a person's fetch, of similar nature to the German doppelgänger, can be seen as a person of undetermined gender, an animal, or other dissimilar being.