There's a rare psychiatric disorder called Capgras Delusion, where you think your family and friends have been replaced by imposters.
There's the part of your brain that recognizes faces, and the part that stores memories and feelings associated with a person. In a normally functioning brain, when you see a person you recognize, your brain looks up the feelings associated with that person. In Capgras, that process fails, so when you see someone you know and love, you get this strong sense that it just doesn't feel like that person. So your brain reconciles that disconnect by concluding that the person must be an imposter or duplicate.