Well the thing is, w4r's reference to other abilities going through and draining IS is the same issue but more of a hinderance than a help.
Duelist often has damage glitches when powerless or not where Dual Pistol Excecutions go through to quickly, not matching up with the 4 second round of combat and the ability does no damage and drains the IS - it also happens when powerless, drains the IS but does no damage which is right as you're powerless, but it shouldn't drain the IS.