I wish Duality let all weaponskills strike twice for 10 seconds. Basically a Ninja's very own self buff in lieu of Trick Attack. I'm a bit bummed it's just a glorified Life Surge without the heal. Optimal use of Duality would be for Aeo Edge since it would be our hardest hitting attack. Personally though if you know you won't be able to fit an Aeo in then you can spend Duality on a different attack or just wait until you have a good opportunity to use it with Aeo.