Fregoli delusion is a delusional misidentification syndrome which describes an individual’s mistaken belief that different people are in fact the same person in disguise who is able to change their appearance. Misidentification syndromes all involve a belief that the identity of a person, object or place has somehow changed or has been altered.
The Fregoli delusion was first described in 1927 in the paper Syndrome d’illusion de Frégoli et schizophrénie. In it the authors described a case of a 27-year-old woman living in London who believed she was being persecuted by two actors she often saw at the theatre. She believed these people pursued her, taking the form of people she knew or met.
The Fregoli of the delusion’s title refers not to the authors, but more modestly to an Italian actor, Leopoldo Fregoli, who was renowned for his ability to make quick changes of appearance during his stage act. In the Fregoli delusion the sufferer often thinks that they are being persecuted by the misidentified person.
Whereas the similar Capgras syndrome involves an under-identification of people and places and it has been postulated that this syndrome results from the inability to match current experience to autobiographical memories. Fregoli delusion conversely involves over-identification and a seeming confabulation of resemblances between the misidentified entity and the original, so this explanation is not as satisfactory.
I found this paper offers which offers half an explanation:
A partial answer … may come from Rapcsak and colleagues who described a patient without prosopagnosia who displayed false recognition (over-identification) of faces following the surgical removal of a right pre-frontal lesion. They attributed the patient’s pattern of impairment to an intact reflexive face-recognition system but an impaired reflective or strategic face-processing system, leading this patient to mistake an unknown face for one in memory. This kind of defect might explain some instances of visual over-identification of faces. This account still does not explain selectivity, refractoriness, delusional nature, or multimodality.
(June 2018 review – broken links removed)