Doppelganger

Doppelganger is a german word 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 seen 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.