In the episode In a Mirror Darkly II, Mirror Archer is apparently experiencing visual and auditory hallucinations of Prime Archer. It struck me today that perhaps both Archers were born with schizophrenic genes, but only the Prime version had it treated as a younger person. Then again, the recent DSC ep 3x04 (not a spoiler) suggested that perhaps mirror universe humans have a variant genome predisposing them to be violent lunatics. So maybe it is unique to Mirror Archer after all.