1. A ghostly double or counterpart of a living person
2. Alter ego; double
This word dates to 1830 and comes from German Doppelgänger, which is literally "double-goer" and originally had a ghostly sense. Sometimes it is half-Anglicized as doubleganger.
The fact that doppel and double sound similar and have similar meanings seems to be an accident of language change because English double dates to the early 13th century and comes from Old French doble which derives from Latin duplus. Duplus is a combination of duo- + -plus ("two" + "plus"). Duo and a plethora of other languages' words for "two" ultimately derive from Proto-Indo-European *duwo.
Gänger and English gang, on the other hand, are definitely related. Old English gang ("a going, journey, way, passage") came from Proto-Germanic *gangaz, which is also the forebear of the gang that appears in Old Saxon, Old Frisian, Danish, Dutch, Old High German, and German. *Gangaz derives from the Proto-Indo-European base *ghengh- ("to step") which is also the forebear to Sanskrit jangha ("shank"), Avestan zanga- ("ankle"), and Lithuanian zengiu ("I stride"). Around the mid-14th century gang meant "a set of articles that are usually taken together in going", as in a set of tools or something similar. By the 1620's its usage had extended in nautical speech to mean "a company of workmen" which is still reflected in the words gangway and gangplank. About a decade later gang was being used to refer to "any band of persons traveling together" with a negative overtone. This is, of course, one of the senses that survives in 20th and 21st century Modern English with street gang, gangsters, and the like.
|My doppelganger. Yeah, right...|
Etymologies come from the Oxford English Dictionary and/or Etymonline.com