We've all met the people who are so intolerably desparate to win duels that they use first aid during them. *Cough Warlocks Cough*
Have no fear (pun) a simple way to prevent this is when you see the countdown for the duel to begin, quickly use some crappy, low-grade bandage on them, which will trigger the "Recently Bandaged" response, making them unable to first-aid themselves for 1 whole minute. Give 'em hell.
Good luck, people.
**Edit**
It seems many people have been commenting on my crack against Warlocks. All in good fun, folks.