Capgras syndrome

noun

Cap·​gras syndrome ˌkap-ˈgrä- 

variants also Capgras delusion

: a delusional condition characterized by the false belief that known individuals (such as family members) have been replaced by doubles or impostors

The ability to identify (which was intact) was not accompanied by the appropriate feeling of warmth and familiarity, and it was this contradiction that led to the logical though absurd conclusion that she was a "duplicate." (This condition, which can occur in schizophrenia, but also with dementia or delirium, is known as Capgras syndrome.)

—Oliver Sacks, The New Yorker