when his highschool bully returns, a stuck-in-home-town loser must prove his competence by kicking his ass in the duel he proposed and save his family name

Suggestion:? the inciting incident is the new girl in town whom he falls in love with.? So he has to stand up to the bully -- instead of run away -- if he's going to impress her.

"Whatever a man does, he does for his ideal woman." -- George Balanchine

fwiw

As variable said.

And doesn't he have to prove his courage rather than his "competence"?

Also if it's the victim who proposes the duel, what motivated him to do it?? ?If the backstory is that the bullying has been an ongoing problem, what difference does it make that the bully has returned?? Doesn't his return just resume the same ol' same ol' -- the status quo? What specific event? has the motivated the victim to propose a duel?

so it is "the bullied"...who proposed the duel? That's unclear in the logline

sounds like "Eagle vs Shark" (2006) to me
A film by taika waititi, about two social misfits and how they find love through revenge on a highschool bully..