Doubling is caused by the disconnector not reliably "catching" the rear trigger shelf when the recoiling bolt recocks the hammer. This means that the disconnector "bite" on the rear hammer shelf is too shallow. Pop the lower off of you gun and pay with it. When you are doing this, always catch the hammer with a thumb when firing to make sure it does not slam forward and hurt something as you "fire" it.
As I look at pictures of this trigger, it functions similar to the stock trigger, but allows for finer tuning. I have made similar changes to my stock trigger by using shims for over travel (front of the trigger - 0.010" thick steel), pre-travel (rear of the trigger - 0.032" brass) and the front of the disconnector foot (0.016" brass). There are details about this in earlier threads.
In my case, doubling was caused by the disconnector not having enough "bite" on the rear of the hammer shelf. With the trigger pack pulled out, hold the hammer down with one thumb, press the trigger all the way back and watch how the disconnector at the rear of the trigger assembly "catches" the rear of the trigger shelf.
In my case, the over travel compensation was fixed with a shim and I never messed with it. I filed down the 0.016" brass shim under the front of the disconnector foot in order to rotate the disconnector forward enough to get enough "bite" on the rear shelf. Smaller disconnector shim = more "bite", but also a longer reset. I found that the disconnector needed a "bite" of 1/3rd to 1/2 of the rear trigger shelf in order to reliably "catch" the hammer after firing. If this "catch" action is not reliable, doubling will happen.
This trigger is not using fixed shims but instead has an adjustable over travel. If you adjust the front of the trigger down a bit (i.e., give up a little bit on the over travel), the whole trigger pack can now rock forward a bit further (including the disconnector hook) when the trigger is pulled. This will allow the disconnector to get a bit more bite when the trigger is pulled and will perhaps stop the "doubling". The only alternative I would see would be to grind the front disconnector foot down a little bit to increase the disconnector "bite", but I would hope it does not come to that.
Note that there is a trade between shorter reset and a more reliable hammer "catch" (i.e., no doubling). Making the "catch" (i.e., more disconnector bite on the rear hammer shelf) will make the reset a bit longer.
- DanT Phoenix, AZ