Doppelganger

Doppelganger is a german word (correctly Doppelgänger) that means 'look-alike' and is in myths a paranormal double of a normal person.

Doppelgangers appear in many mythologies across the world and usually are seen as a bad omen or a punishment on someone. In some traditions seeing the doppelganger of a friend or relative could be taken a warning of illness or death to come; grim times indeed.

Because mythology is a free idea bucket for many settings, doppelgangers have appeared in various forms in nearly every franchise possible in some form.

Warhammer Fantasy and 40k

The daemons in the GW settings are terribly fond of appearing as someone's resemble to cause confusion or to further their own plans through misdirection. It can be hard if not impossible to tell if someone is the real thing or not.

The Changeling, daemon of Tzeentch, is the ultimate Doppelganger and uses this ability to troll everyone across the settings. The problem for him is he has changed so much he can't even remember his original form now and that just grinds his gears.

Dungeons & Dragons

In Dungeons & Dragons, doppelgangers have a long history. Described as resembling almost elf-like androgynous grey-skinned beings, doppelgangers are portrayed as skilled shapeshifters with fundamentally lazy temperaments; believed to have originated from some evil wizard's experiments to create the perfect assassin and puppet ruler, they have since gone wild and use their skills to try and replace powerful individuals so they can lead lives of luxury. Consequently, they still tend to end up employed as assassins and "veils" for evil wizards and similar types.

In AD&D, there are several kinds of doppelganger, the two main variant strains being the Greater Doppelganger (who can absorb all of a person's memories by eating their brain) and the more magical Ravenloftian Doppelganger (who has mind-reading powers and minor illusion powers).

In Eberron, doppelgangers are believed to be the ancestors of the Changeling race.