I go back and forth on the numbers of Duress depending on the metagame, Duress is often competing with Cabal Therapy for slots. Where Duress really shines is against opposing combo, prison, and the card Counterbalance. There are match-ups where you can't afford to be wrong with your naming selection with Cabal Therapy, these are generally the match-ups where I want Duress. The weakness of Duress is not being able to discard cards such as Thalia, Guardian of Thraben. In a more "fair" metagame, I could see not playing Duress at all but in "degenerate" metagames, I'd want more copies of Duress over Cabal Therapy.