The Capgras delusion is a rare delusion that one or more of your loved ones have been replaced by identical impostors. It’s most often seen in people with schizophrenia, but can also occur in people with brain damage, dementia, even diabetes. It’s thought to be caused by neurological damage (like brain lesions).
Basically, the human brain seems to have two separate pathways for facial recognition: one pathway for strictly visual recognition, and another pathway for generating an emotional response. In people suffering from the Capgras delusion, it’s thought that the visual pathway remains intact, but the pathway for an emotional response somehow gets severed; people interpret their lack of emotional connection when seeing their loved ones to mean that they are in fact not their loved ones, but doppelgangers. More recently it’s been linked to a failure of patients’ memories of what their loved ones are like (as opposed to what their loved ones look and sound like; those kinds of “external” memories remain intact). Apparently people suffering from Capgras delusion also sometimes have difficulty with their episodic memory, too.