A person falsely perceives that his close friend has been replaced by an exact double. This phenomenon is referred to as:

Capgras syndrome: false belief that a familiar person or place has been replaced with an exact duplicate. Fregoli delusion, or the delusion of doubles, is a false belief that different people are in fact a single person who changes appearance. Cotard syndrome is a false belief that they are already dead, are putrefying, or have lost their blood or internal organs.