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The Capgras delusion (or Capgras syndrome) is a disorder in which a person holds a delusion that a friend, spouse, parent or other close family member, has been replaced by an identical-looking impostor. The Capgras delusion is classed as a delusional misidentification syndrome, a class of delusional beliefs that involves the misidentification of people, places or objects. It can occur in acute, transient, or chronic forms.
The Capgras delusion is named after Joseph Capgras (1873-1950), a French psychiatrist who first described the disorder in 1923 in his paper co-authored by Reboul-Lachaux,on the case of a French woman who complained that corresponding "doubles" had taken the places of her husband and other people she knew.
Above words are from wikipedia: http://en.wikipedia.org/wiki/Capgras_Syndrome