Upkeep on Duel Servers
I would like to suggest two kind of "duels";
1) A training - both combatants receive tiny ammount of XP, and no gold, without a risk of breaking any equipment. This mode would be fit for, well, personal training, practice and friendly sparrings.
2) A (proper) duel - a winner gains a significant ammount of gold and xp, while defeated person suffers a normal chance of breaking equipment, without earning any gold or "experience".