A doppleganger is a term used in Gothic fiction to refer to the ways in which characters can function as a kind of ghostly twin for another character. If we wanted to compare Macbeth and Duncan using this concept, it would be clear that the way in which Macbeth functions as a doppleganger is through his craving for power. In a sense, we could also argue that Duncan and Macbeth act as foils in this play, as is shown by the naivety of Duncan and the way in which he is depicted as being such an innocent in walking into the trap that Macbeth and Lady Macbeth set for him. Note how Duncan reveals this worrying trait about himself in Act I scene 4 when he talks about how he was betrayed by the former Thane of Cawdor:
Duncan himself admits that he finds it impossible to "find the mind's construction in the face" or to see the true character and heart of a man through the way they appear. Macbeth therefore could be described as his doppleganger in the way that Macbeth pursues and craves the crown, but definitely not through any similarity of character.