The Capgras delusion is an extremely rare disorder that leads somebody to believe at least one person close to them has been replaced by an imposter. It is impossible to convince somebody with Capgras that this is not the case through reason and logic.
While to many of us this condition exists only in the abstract, a piece of trivia to be prodded and poked with thoughts and theories, it is a horrifying reality for anyone living under the delusion. It must be truly traumatic. Imagine believing that you know – not think, but know – that a loved one such as your daughter, father, husband, wife, brother, or sister has been inexplicably ripped from your life, and an identical-looking person is for some reason pretending to be them, and seemingly getting away with it.
Conversely, imagine that (for example) your parent or child has developed Capgras delusion, and you are the one they believe has been replaced. Your presence will make them angry and upset. Just being in the same room would cause your loved one severe distress.