<unk>  is an umbrella term for a group of<unk>  disorders that occur in the context of mental or neurological illness. They all involve a belief that the identity of a person, object or place has somehow changed or has been altered. As these delusions typically only concern one particular topic they are also fall under the category called monothematic delusions.
This syndrome is usually considered to include four main variants1:
The Capgras delusion is the belief that (usually) a close relative or spouse has been replaced by an identical-looking impostor.
The Fregoli delusion is the belief that various people that the believer meets are actually the same person in disguise.
Intermetamorphosis is the belief that people in the environment swap identities with each other whilst maintaining the same appearance.
Subjective doubles, in which a person believes there is a doppelgänger or double of him or herself carrying out independent actions.
However, similar<unk>  beliefs, often singularly or more rarely reported, are sometimes also considered to be part of the<unk> . For example:
Mirrored self-misidentification is the belief that one's reflection in a mirror is some other person.
Reduplicative paramnesia is the belief that a familiar person, place, object or body part has been duplicated. For example, a person may believe that they are in fact not in the hospital to which they were admitted, but an identical-looking hospital in a different part of the country, despite this being obviously false2.
Syndrome of<unk>  companions is the belief that objects (such as soft toys) are sentient beings3.
Clonal pluralization of the self, where a person believes there are multiple copies of him or herself, identical both physically and psychologically4.
There is considerable evidence that disorders such as the Capgras or Fregoli syndromes are associated with disorders of face perception and recognition. However, it has been suggested that all<unk>  problems may exist on a continuum of anomalies of familiarity5, from déjà vu at one end to the formation of<unk>  beliefs at the other.
Delusional misidentification syndrome
Battered person syndrome
Suicide crisis
Depersonalization disorder
Munchausen syndrome by proxy
Dissociative identity disorder
Dissociative disorders
Substance or medication induced psychotic disorder