Doppelganger

Doppelganger is a German word (correctly Doppelgänger) that means 'look-alike'; in myths, it is 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.

Still, unlike many D&D monsters, doppelgangers aren't inherently, or even commonly evil, just lazy, empathy-impaired jerks. (X5 brings in the Mujina for that.) In AD&D, there are several kinds of doppelganger; the most notable is the Greater Doppelganger, which can absorb all of a person's memories by eating their brain.

Ravenloft is home to the more malicious Dread Doppelganger, which has far more refined mind-reading powers and can use magical illusions to further supplement its disguises. But unlike normal doppelgangers who can theoretically live outside of normal society, they cannot reproduce unless they mate with a humanoid (typically as a male, as a "pregnant" Dread Doppelganger cannot shapeshift). They hail from the domain of Peridon, whose Darklord is himself a Dread Doppelganger.

In Eberron, doppelgangers are believed to be the ancestors of the Changeling race. They form the leadership of the Tyrants, a gang that controls much of the prostitution in Sharn. How a gang that's mostly legal enterprise wound up being called the Tyrants is anyone's guess.