I had that problem, and it was very common (about a 50% chance per shot, possibly more), and it affected my accuracy A LOT because when two bbs would fire, they both drop faster than the rest.
THE FIX: All I did to fix this for me, was I took down the down, cleaned the feeding tube a bit, closed it up, shot some silicon lube up the feeding tube and into the nozzle, sat the gun on its stock so the lube would run into the gear box, down the nozzle, etc. I took it outside and fired off a good many shots to purge the lube from my barrel and fix the accuracy issues that come with lubing. To my surprise, I noticed the double feeding was practically GONE. I switched it to semi, let off almost an entire hi cap mag and had just about two misfeeds, which occurred strangely after I adjusted the hopup. This is with the KWA Hicap mag. Hope this helps .
Edited by xXxGruntxXx, 17 July 2008 - 12:31 PM.