I think the game should inform the player when he hits an already broken limb of an NPC.
And maybe report the damage as 0.
And maybe add limb damage indicators for NPCs, just to see how beat up their various body parts are.
And maybe put a hard cap on how often you could possibly hit a broken body part. I mean, if you have 4 broken limbs, do you have like 4/6 chance of being hit to those locations?