A doppelgänger is the ghostly—in some cases, the physical—double of a living person. The word "doppelgänger" is a loanword from German, and the word derives from Doppel ("double") and Gänger ("goer").
The word is also used to describe a phenomenon whereby one catches his own image out of the corner of his eye. In some traditions, seeing one's own doppelgänger is an omen of death. A doppelgänger seen by a person's friends or relatives may sometimes bring bad luck, or indicate an approaching 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 dopplegänger.
Many cultures around the world have versions of the doppelganger and the phenomenon of it is almost as ubiquitous as the werewolf or vampire. 

I type all this out only as a warning to a dear friend.
Jen Thompson, do not come to Los Angeles! Your doppelganger is working in a kitsch shop called Duck Soup and if you bump into her here you will cease to exist!
Seriously, I actually wondered why you were in CA the first few times I saw her.