Why bother with other spells?
If the purpose of a duel is to eliminate the opponent, the Avada Kedavra spell is all you need. Why do these witches and wizards even bother with fire/ice/water/glass attacks, disable spells, stun-lock spells, knock back spells, charm spells.... seriously, just AK them and be done with it.
If I were in a wizard duel, I'd be shouting AK so much green bolts would shoot out of my wand like a machine gun.
If I don't want to kill my opponent, then the mind control curse instead. But really, what's the point of any other spell if it's a duel to the death?