Another area to inspect is the sear and sear pivot pin on which ever is the second barrel to be fired.
I have a early Hunter Arms that is one of the XX ungraded guns in 10 gauge #31,534 which was firing both at once and since front trigger is normally fired first I knew issue was with left lock and first checked for lock to stock interference that might cause lock to trip and then inspect lock sear to hammer fit and condition of the sear pin fit in the plate.
In my case it was worn pin with too much play in the plate. You need to take lock apart and drive the pin out of sear then measure unworn section that is inside sear for correct size. For mine I was able to match with drill rod from Ace Hardware.
You can retest results and save your shoulder and finger by putting a unload primed shell in the second shot barrel that way if it still fires both you save your self and the stock from possible damage.
Hope this will be of some help.