Stop thinking of Duality as something unique. Instead, think of it more as NIN's flavor of DRG's Lifesurge. The only reason they SPECIFICALLY say it doesn't double the bonus is because instead of just giving NIN their own 'next WS crits' was to make it seemingly set apart from DRG. In reality, NIN and DRG are very similar. Both get a 10% slashing/piercing ability at 38, both get a 360pot skill (Raiton/Full Thrust). Everything balances out. Start thinking of Duality as a functional equivalent and NIN's version of Life Surge--to be used on your highest potency WS (Duality doubles the potency compared to LS which just makes it crit;however Duality has a longer recast therefore it balances out in the end). They simply added the clarification that it doesn't double your bonuses so people don't get any silly ideas.