In modern vernacular it is simply any double or look-alike of a person. It also describes the sensation of having glimpsed oneself in peripheral vision, in a position where there is no chance that it could have been a reflection. Doppelgängers often are perceived as a sinister form of bilocation and are regarded by some to be 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. In Finnish mythology, this is called having an etiäinen, i.e., "a firstcomer". In Ancient Egyptian mythology, a ka was a tangible "spirit double" having the same memories and feeling as the original person. In one Egyptian myth titled "The Greek Princess," an Egyptian view of the Trojan War, a ka of Helen was used to mislead Paris of Troy, helping to stop the war. In some myths, the doppelgänger is a version of the Ankou, a personification of death. (Source: Wikipedia)
