I can't be bothered to look it up at the moment, but I know that Richard II starts with a planned duel between Aumerle (I think) and Bolingbrooke. If Greg hasn't cracked it for you already, you might try having a look in there. There must be loads of other duels in Shakespeare, but they don't all spring to mind at the moment. Jeannie is probably the one for this. She knows about that stuff.
Romeo duels with Tybalt in Romeo and Juliet. I can't remember whether an actual duel breaks out between Proteus and Valentine in Two Gentlemen of Verona... (The soppy gits kiss and make up anyway - not enough bloodletting for my tastes...) Hamlet duels with Laertes and muffs it, of course.... You have got me going. I'll probably be awake all night thinking about this now!