Double Attack isn't increasing damage in Katars.
Double Attack skill is supossed to boosts the off-hand (left hand) damage of a Katar weapon by (1+2*SkillLV)% ATK.
Steps to Reproduce
- Equip katar in a sin and hit a monster
- Compare calculations damage with calculations from http://calcx.wushuang.ws/
Damage is equal to calculations without Double Attack mastery but damage isn't increased for leveling the skill.