I have a 10 gauge Zabala that started doubling on me, that will flat wake you up!!
Now this sounds crazy I know but it is what happened to me, on my gun the sear has a long arm on it that reaches back to the trigger, very simple, somehow, and this is the crazy part, that sear shifted so it made contact with the trigger before it was fully engaged, so the recoil from the front trigger would set off the rear trigger, I checked everything, nothing loose, nothing broken, nothing lodged under the trigger making it ride high, just crazy, I "adjusted" the sear and the gun has worked fine ever since, that has been at least 20 years.
Any way you should at least look to see if something is making the trigger rid high, or making the sear arm ride low.
On mine when I checked it the sear would only engage the tumbler about half way, and like I said the rear arm of the sear would rest on the trigger. And yes it was the rear trigger!
Good luck!
TM
