A Doppelgänger or a double is a duplicate or a look-like of a person, whether archived biologically (e.g.: Project Helix) or cosmetically (make-up). They are often placed strategically during espionage situations when one intelligence agent needs to assume the identity of a target individual. The target may or may not have been replaced or killed prior the placement of the double.
Project Helix was an important technology which allowed for genetic manipulation of a person's DNA to assume the role of another.