Such things are really boring. I've seen a few myself, and they've never really been satisfying. Where a multi-person combat gets all the players involved, and means that teamwork is an important and fun part of the fight, duels tend to be lacking in such elements, and tactics tend to go out on the window in favour of "let's see who can roll higher".
My thoughts on solutions for the lack of tactics/prevalence of "who can roll higher?":
- Everybody has to move at least one square (or whatever) every round, and the terrain on which the duel takes place should be at the very least, varied in elevation, if not studded with things that can be used for cover. Sword fights in movies are never static
- Raising the stakes should be used.
Each player, at the start of every turn, gets to assign a small bonus to his favoured fighter. If you like an in-game explanation for this, it could represent cheering him on. Pick one of the following:
- +1 damage on all attacks
- +2 AC
- +3 temporary HP
I suspect the list above could do with expanding, but I think it's a good start.