Zael identifies it by watching closely to a mirror and sees that it makes an error when it's copying him. But while fighting it copies Zael near perfectly. They use Yurick's magic to make it move differently to allow Zael to strike it.
After a while the mirror breaks and three more Doppelgangers appear. During most of the fight Zael and his friends confuse their allies with the doppelgagers. At the end the Doppelganger doesn't know who to copy and perishes.