Riposte, Duelist, and Attacks of Opportunity all consume a hidden third combat resource - your Reaction. Like your Action, you only get one per turn, so even while all are toggled on your character can only make one Attack of Opportunity, Riposte, or use their Duelist feature (prioritizing the first one to trigger).
Some examples:
An enemy moves away from your character, thereby triggering your Attack of Opportunity. Your character consumes their reaction to swing at them! Now, until it is their turn again, your character cannot make another Attack of Opportunity, nor use Duelist or Riposte.
Next turn, an enemy approaches and swings at you. Your character uses their Duelist feature to raise their AC by two, causing the attack to miss. Their reaction is now consumed, preventing them from using Riposte.
For this reason, I suggest activating Riposte or Duelist, not both. With both active, Riposte will never trigger, because Duelist will always consume your Reaction first.
Last edited by fishworshipper; 13/10/20 08:22 AM.