Playing Duds on Swan's Beak would be doubles and would trigger another two dice to be rolled.
If a player successfully rolls doubles on the first Swan's Beak roll and then rolls again, if the opponent plays Duds or Diabolical on the Swan's Beak roll It would only effect the two most recent rolls and Duds would trigger another set of two die to be rolled. Swan's Beak's initial attack triggers another current roll, and the two most recent dice are that players' new current roll. No matter how many times Swan's Beak triggers Diabolical/Duds would only effect the two most recent dice. The total amount of the Swan's Beak roll is still considered the initial attack, even though only the two most current dice are considered the current roll.