Hi there! Sunder was one of those rules left for Referee interpretation and discretion. Personally, I most often treat a Sunder attempt just like an Attack, except it’s targeting an enemy weapon or piece of armor instead of their vitals. You can assign hit points to weapons/armor based on their Load or your judgment—or just do what I do and have a successful Sunder automatically destroy the target weapon/armor on hit.
Under this interpretation, a Duelist who has taken the Advantage: Sunder feat automatically has advantage on the Sunder attempt, barring other circumstances that might grant them disadvantage.