1. In psychiatry, a delusion that a double of a person or place exists elsewhere.

It is related to other defects in recognition and suggests organic disease in the non-dominant parietal lobe of the brain.

2. A ghostly double of a living person; especially, one that haunts its bodily counterpart.
3. An alteration in the perception of self so that the usual sense of one's own reality is temporarily lost or changed.

It may be a manifestation of a neurosis or another mental disorder or can occur in a mild form in normal people.

4. A reduplicative paramnesia consisting of the delusion that a double of a person or place exists elsewhere.

The phenomenon suggests non-dominant parietal lobe pathology and is related to other disturbances of recognition.