A doppelgänger is an exact duplicate of a person, often turning out to be an evil twin. In the case of a person already being evil, the twin will always be more evil, pwning the original.
They are thought to have been created by malevolent color inkjet printers; there is no other explanation. Why this is is anyone's guess, but it may have to do with magic spells cast by equipment manufacturers to make people buy their ink cartridges only. Others think that The Matrix's Agent Smith or race car driver Vanellope von Schweetz are out to have a laugh, accessing printers via their power cables.
In any case, doppelgängers are totally obnoxious creatures, following your every move, farting when you fart (and being equally smelly), picking your nose when you do it. They will be the one to smash their shopping cart into yours as you come around a corner in a supermarket. Your phone bill will be doubled. No one can pwn you better than yourself.
The worst case is of doppelgängers of twins, triplets, etc. Total confusion reigns unless they all get together to form a criminal street gang. Then there will always be one or more of them somewhere conspicuous to provide alibis to prevent any of them from being arrested. Naturally, these groups are known as doppelgängerbangers.