Doppelgänger

A doppelgänger (, literally "double-goer") is a paranormal phenomena in which a living person is seen in more than one place at once, usually seen as a harbinger of bad luck. In modern times, the term twin stranger is occasionally used. This is distinct from a "dead ringer", another person with physical characteristics of uncanny similarity. The word "doppelgänger" is often used in a more general and neutral sense to describe any person who physically or behaviorally resembles another person.

Mythology
English-speakers have only recently applied this German word to a paranormal concept. They are also known as (false) arrival apparitions. Doppelgangers appear in many cultures.

Scandinavian folklore
In Scandinavian folklore, a vardøger (vardyvle or vardyger) and is a ghostly double who is seen performing the person's actions in advance. In Finnish folklore, it's called having an etiäinen,  "a firstcomer", thought to be a kind of haltija.

Etymology
Vardøgr is a Norwegian word defined as "premonitory sound or sight of a person before he arrives". The word vardøger is probably from Old Norse varðhygi, consisting of the elements vǫrð, "guard, watchman" (akin to "warden") and hugr, "mind" or "soul". Originally, vardøger was considered a fylgja, a sort of guardian spirit.

Irish folklore
In Irish folklore, it's called a fæcce (fetch) and is generally taken as an omen of the person's impending death, though John and Michael Banim report that if the double appears in the morning rather than the evening, it is instead a sign of a long life in store.

Ancient Egyptian folklore
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.

Examples
Emilie Sagée was a 19th century teacher in Latvia who had a doppelganger appear numerous times to her students, but never to her. At first it would merely copy her movements, then later act out things she wished she were doing (such as teaching her sewing class instead of gardening). The doppelganger was always silent and didn't react to stimuli. Students who tried to approach the apparition found that the air around it felt like "thick fabric". She was eventually asked to leave because it frightened the students.

Abraham Lincoln is said to have seen his doppelganger shortly after being elected to his first term. He laid down on a couch to rest and glanced at a mirror, in which he saw two faces looking back at him. He was sure the second face was his, but it appeared pale. His wife interpreted this as an omen that he would be elected president twice, but die in his second term. Sure enough, he was assassinated soon after his second election.

Catherine the Great encountered her doppelganger in 1796 when she was awoken by servants who told her they'd seen her walking into her throne room. Upon investigating, she found her likeness sitting upon the throne. She immediately ordered her sentries to shoot at it, which had no effect. It eventually faded from sight, and she died shortly thereafter.

Izaak Walton claimed that English metaphysical poet John Donne saw his wife's doppelgänger in 1612 in Paris, on the same night as the stillbirth of their daughter.

German playwright Goethe described an experience in his autobiography Dichtung und Wahrheit in which he and his double passed one another on horseback.

Explanations
Explanations for the phenomena vary, from "reality glitches" and alternate realities bleeding into ours, to parasitic demons which can impersonate a host.

Other sources

 * Davidson, H.R. Ellis (1965) Gods and Myths of Northern Europe  (Penguin Books) ISBN 978-0140136272
 * Kvideland, Reimund; Henning K. Sehmsdorf  (1989) Scandinavian Folk Belief and Legend  (University of Minnesota Press) ISBN 9780816615032
 * McKinnell, John (2005) Meeting the Other in Norse Myth and Legend  (D.S. Brewer, Cambridge) ISBN 978-1843840428
 * Orchard, Andy (1997) Dictionary of Norse Myth and Legend (Cassell & Co) ISBN 0-304-34520-2
 * Pulsiano, Phillip; Kirsten Wolf    (1993)  Medieval Scandinavia: An Encyclopedia (Routledge Encyclopedias of the Middle Ages) ISBN 978-0824047870
 * Simek, Rudolf; translated by Angela Hall (2007) Dictionary of Northern Mythology (D.S. Brewer, Cambridge) ISBN 0-85991-513-1