When we used to do duels, it hasn't come up for a while, we would have them square off, roll intiative and go at it. I did one once during 3.0 and what we did was to give each a full round as normal to cast a spell, move, whatever. Spells only, no items. Then, in addition to casting a spell, you were given a readied action to counter a spell, otherwise, in the mid to higher levels, the first to go wins, this made it more interesting. We made a list of unhonorable spells to cast during a duel, invisiblity of any sort was forbidden, along with save or die. We tried to make it fun, not whoever goes second dies.