Back in the late 1950's I had a pre-WWI 12 gauge J Stevens Arms & Tool Co double which began doubling if you pulled the rear trigger first. I pulled the stock to investigate, turned out the left sear tail sat a bit lower than the right one. There was enough wear to the sear pin/sear hole engagement the left sear tail would catch on the right one as it was lifted firing both barrels simultaneously. I did not give it a proper fix of re-doing the sear pin/sear holes. I simply cut a short section of spring from a ball point pin & placed it on the pin between the sears which kept them pushed apart, never had another problem with it. My first gunsmithing project.
It was still that way a number of years later when I foolishly traded it off. I say foolish, not because of any intricate value of the gun, but it was the first shotgun I had personally owned, first double I had ever sot & was the gun which started me down the path of using only SxS doubles. all value was purely sentimental, but I have regretted getting rid of it many times over the years.
When this first doubling occurred I had just shot at a squirrel in a tall tree, so hit the back trigger. I noticed a loud boom, which I attributed to an echo from being under the thick leaves, but noticed no undue recoil. It was a 30" barreled gun of at least 7 1/2 lbs weight, maybe more & I was only firing 3DE with 1 oz of #6 shot. The next time I pulled the rear trigger it snapped & on opening the shell was fired, but not then. I went back to the tree where I had shot that first squirrel & sure enough found the two fired empties there, was not reloading yet.
There are many little thing which can cause doubling which can be determined only by a through investigation. Fortunately "Most" of them are rather easily fixed, though not all are.
