On the page for vanaduke it states:

"Stun will cause him to double-attack. If he is stunned in the middle of one of his moves, he will perform it instantly ahead of his animation, and again once his animation is completed. A mace swing will create double the ceiling debris and lava spawn, and a dash will hit you before he's even there. He will also double-summon slags, which will kill the first set and give a chance at free hearts. "

However I recently decided to try my Callahan on him (risking my life and, without their knowledge :P, the rest of the group's lives) and did stun him a few times but I don't think I ever saw him do a "double-attack" (we defeated him), was this patched or did I just not notice it happen? If it was fixed the line should be removed from the known issues for him.