"Doppelganger" is German for "double walker" - a shadow self that is thought to accompany every person. Traditionally, it is said that only the owner of the doppelganger can see this phantom self. Occasionally, however, a doppelganger can be seen by a person's friends or family, resulting in quite a bit of confusion.
The Doppelganger is most often viewed as a bad omen, and sometimes seen in itself as being a sinster being, like the preverbial 'evil" tiwn, it is most commonly beleived that they only show up as signs of death to come, or other dangers. 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 doppelgänger he is doomed to be haunted by images of his ghostly counter-part. Other folklore says that when a person's dopplegänger is seen, the person him/herself will die shortly. It is considered unlucky to try to communicate with such a doppleganger.
Doppelganger can also be used to descibre the phenemoen known as biolocationg, in which a person, sometimes intentionaly, or in some cases subconciously project a perfect image or double of themselves, in which they can acutally be in two places at once, as the double is indisqunsiable from the real person, and in some cases is beleived to appear of physcial form, and being able to interact with people and the physcial world.