Ah, I'd expected KO to do better than that.
You could argue that a mirror image only goes down when you manage to strike an actual hit at an enemy, then you'd know which of the ones is not an illusion. The Eye Shield works passively (I assume that the vengeful spirit in it is invisible), all those mirror images are attacking the wearer and then stumble back with an eye gouged out simultaneously. No way telling the real one this way.