A doppelgänger is a non-biologically related look-alike or double of a living person, sometimes portrayed as a ghostly or paranormal phenomenon and usually seen as a harbinger of bad luck. Other traditions and stories equate a doppelgänger with an evil twin. In modern times, the term twin stranger is occasionally used.The word “doppelgänger” is often used in a more general and neutral sense, and in slang, to describe any person who physically resembles another person.
English-speakers have only recently applied this German word to a paranormal concept. Francis Grose’s, Provincial Glossary of 1787 used the term fetch instead, defined as the “apparition of a person living.” Catherine Crowe’s book on paranormal phenomena, The Night-Side of Nature (1848) helped make the German word well-known. However, the concept of alter egos and double spirits has appeared in the folklore, myths, religious concepts, and traditions of many cultures throughout human history.
In Ancient Egyptian mythology, a ka was a tangible “spirit double” having the same memories and feelings as the person to whom the counterpart belongs. The Greek Princess presents an Egyptian view of the Trojan War in which a ka of Helen misleads Paris, helping to stop the war.[citation needed]. This is depicted in Euripides’ play Helen. In Norse mythology, a vardøger is a ghostly double who is seen performing the person’s actions in advance. In Finnish mythology, this is called having an etiäinen, “a firstcomer”. The doppelgänger is a version of the Ankou, a personification of death, in Breton, Cornish, and Norman folklore.
(From Wikipedia – https://en.wikipedia.org/wiki/Doppelganger