They've caught a Body Snatcher or Symbiote, or are Sharing a Body, and you can tell, because they're obviously switching between minds repeatedly. Often accompanied by changes in manner, posture, voice, or eye color. Those not aware of the circumstances and not sufficiently well acquainted with the originally one-minded character may not notice the change, leading to much opportunity for hilarity to ensue. Having arguments with yourself is strictly optional, but again, hilarious. Examples of Flip Personality include: