Capgras syndrome is a disorder in which an individual holds the delusional belief that a person they are familiar with is in fact an impostor who has replaced the real person. The syndrome has been observed in individuals with brain lesions and neurodegenerative disorders, as well as people diagnosed with schizophrenia.
Capgras syndrome was named after the French psychiatrist Jean Marie Joseph Capgras, who, together with fellow psychiatrist Jean Marie Joseph Reboul-Lachaux, first described the disorder in 1923.
Source: Capgras, J.; Reboul-Lachaux, J. (1923). “Illusion des ” sosies ” dans un délire systématisé chronique”. Bulletin de la Société Clinique de Médicine Mentale. 2: 6–16. (Link to PDF file).