Each face is unique through the anatomical variation present in the bones of the Viscerocranium.

http://en.wikipedia.org/wiki/Face

The pattern of specific organs, such as the eyes, or of parts of them, is used in biometric identification to uniquely identify individuals. The shape of the face is influenced by the bone-structure of the skull, and each face is unique through the anatomical variation present in the bones of the viscerocranium (and neurocranium).