Capgras syndrome

Capgras syndrome (also known as Capgras delusion) is a psychiatric disorder in which it's victim believes that one or more people (usually close family members or dear friends) in their lives have been replaced by a doppelganger. The delusion is named after the French psychiatrist Jean Marie Joseph Capgras, the first known victim of the disorder.

Symptomology
Victims of this disorder hold a strong delusion that someone, a family member, a friend, or even something as simple as a pet, has been replaced by an identical imposter. This delusion can be mild and only cause the victim to misperceive only one relative. Or the delusion can be so strong that they believe everyone around them is an imposter, likely with malicious intent towards them or the 'real' ones they've replaced. This can lead to the victim hurting themselves or others (though actual killing as a result of the disorder is rare). The disorder is most commonly caused by other mental disorders like Schizophrenia or Dementia or a traumatic brain injury.

On Criminal Minds

 * Luke Dolan ("Dorado Falls") - A PTSD-stricken Navy Seal whose Capgras was triggered after a car crash caused by brain damage, leading to a psychotic break. His delusions led to him believing that imposters had abducted his family and were conspiring against him, thus causing him to go on a killing spree, believing he was on a mission to 'save' his family from the 'imposters'.