Here’s my prediction:
Duel happens. The duke pulls some trickery. MC barely wins, but is caught off due to this trickery and her shirt is ripped off, revealing her to be Selena, not Celestine. Everyone is utterly shocked, and furthermore, duke can now blame MC’s bro for deserting, leading to the threatened execution of their whole family, while the MC’s side can accuse the duke for cheating in an honorable duel. Hope I’m somewhat accurate