Just pondering here...
Could not the winner of a duel use the legal argument that he was standing his ground and was in mortal danger of losing his life if he didn't defend himself? Heck, why not bring back gladiators, same reasoning, I had to kill him before he killed me. Following the Zimmerman trial there certainly doesn't appear to be any legal requirement to avoid potentially dangerous encounters in order to invoke stand your ground... why not just go the full monte and make it legal for willing participants to use deadly force against each other.