About duelist class and the feat Canny Defense.
When you put on an armor or equip a shield, canny defense goes off, as expected. But when you dual-wield with rapier in your main hand and another light piercing weapon in your off-hand (dagger for example), canny defense also goes off.
It may be buggy because the class/feat description does not tell anything about this, and ambidextry is a pre-requisite for the class.