Here is my Spiral Knights inventory. If you do a quick search for "Bark Module", you'll find that I already have 24 tokens (and counting) that can only be obtained by defeating the Roarmulus Twins.
Here is my list of achievements.. Apparently I have never defeated the Roarmulus Twins before.
Beating the Roarmulus Twins several times has (as the above suggests) not done anything toward unlocking the achievement, so is there something else that can be done about this? :v
I do have it on mine, but it's at the bottom of the list instead of being next to the other 3 bosses acheivements.
http://steamcommunity.com/id/juances/stats/SpiralKnights?tab=achievements