Great question!
You don't so much as "win" a duel, as the Defending player Concedes the Duel, & tries for a Reprisal.
If the cost of the reprisal is too high, the Victor's victory may be a Pyrrhic one especially if the Spoils aren't what they hoped for. The tension in the game is how long do you hold out for victory in a Duel, how many cards do you hold on to vying for a Reprisal, or do you try to push the other Player to fold before you?