Being that I ascribe to the KISS principle, May I suggest that the reason this happens is that you are not letting the trigger fully reset when rapid firing. That is to say when the trigger hits reset on the rebound after firing a round, your tight grip on the trigger, coupled with the recoil of the 40 S&W round cause a unplanned double tap. It took me well over 200 rounds to adjust to the Glock trigger, I'm very used to a finely tuned 1911 trigger and the Glock trigger gave me fits for a while.
I can still make it happen with my G 23, but it is a planned conscious effort, not an unplanned double tap. MHO