The term is Doppelganger is commonly used to refer to a person's "look-a-like" such as in 1.06 Skin, Dean uses the term Doppelganger to refer to whatever has impersonated Sam's friend Zach. In 7.06 Slash Fiction Leviathans create Leviathan!Sam and Leviathan!Dean - doppelgangers for the boys.
Creatures that can take on the form of a human include:
The ghostly double of a living person, adapted from the German word Doppelgänger (look-alike [literally "double goer"]). The term refers to any double of a person, most commonly in reference to a so-called evil twin. The word is also used to describe a phenomenon in which you catch your own image out of the corner of your eye. In some mythologies, seeing one's own doppelganger is a death omen. A doppelganger seen by friends or relatives of a person may bring bad luck or is an indication of approaching illness.
Dopplegangers in lore
According to ghosts.monstrous.com, Doppelgangers are not merely evil twins, but "ghostly doubles of living people [that] look very similar to the ghosts of the deceased. In some cases a person will come upon his own doppelganger who is typically engaged in some future activity."
In other words, a doppelganger could be considered a spiritual 'remnant' of the future, as opposed to the past (as is with typical ghosts).
- In some mythologies, seeing one's own doppelgänger is an omen of death. A doppelgänger seen by friends or relatives of a person may sometimes bring bad luck, or indicate an approaching illness or health problem.... The doppelgängers of folklore cast no shadow, and have no reflection in a mirror or in water. They are supposed to provide advice to the person they shadow, but this advice can be misleading or malicious. They can also, in rare instances, plant ideas in their victim's mind or appear before friends and relatives, causing confusion. In many cases once someone has viewed his own doppelgaenger he is doomed to be haunted by images of his ghostly counter-part.