Doppler - The Witcher 3

Your package? Your brother picked it up. You know, you two're damn near identical... Hey! Where you running off to?!
– Hanne Kluger, postmaster

Dopplers, also known as vexlings or changelings, are beings able to take on the form of any humanoid or animal they please. This transformation is no mere illusion able to be shattered with a simple spell, but an authentic and complete metamorphosis. This means no protective amulets or witcher medallions will signal a doppler's presence - they emanate the same aura as the being whose form they have assumed. This, combined with their incredible intellect and cunning, would make dopplers supreme assassins or thieves - had not nature endowed them with generous and timid spirits which make them avoid the shedding of blood at all costs.

When backed into a corner, dopplers can present a grave danger by taking on the form of their assailant - and along with it his strengths and abilities. The sum result is to force the assailant to battle against himself - a battle few can win.

Like other relicts, dopplers are extremely vulnerable to silver, and so a silver sword should be used when fighting them. Silver has the additional useful property of forcing a doppler to change form on contact.