Have serious questions here.
I have a customer who owns a SKB/ Ithica 100 SxS 12ga. For those of you who are familiar, you know it's a recoil activated barrel switch system. I have had this in my shop now four times. He keeps having a double firing when he pulls the trigger. I have replaced the sear, modified them and the the sear connector to the point there is no now way both sears are being tripped at simultaneously . I know SKB had a great number of these reported when the 100 came out. Has anyone had similar experiences and if so what was your remedy. I am to the point I think it's the customer pulling the trigger on recoil or "limp shouldering" the shotgun (think 1911 stovepipe) and causing a malfunction.

Have you been able to duplicate it? I bet you can't. I've had several of those shotguns in the shop and never seen a problem like that with that model.

I can't count the number of cases where a customer has brought me their firearm because they were having similar issue e.g. doubling, feeding problems, jamming etc., and after closely inspecting their gun, I walk over to the test-barrel and using quality ammunition the firearm functions flawlessly for me.

One notable case involved a like-new Colt Mustang. Customer was so sure there was something wrong with it, after I'd told him there wasn't, he had me fire two 50-round boxes of ammo while he watched and I could not get it to malfunction, not even once,mane I tried hard to get it to fail.

Another customer brought in a Jennings (normally not even allowed in my shop) he'd owned for 25-years, and had never been able to get it to fire--he said it would never feed a cartridge. Got to wonder why someone would keep a gun that didn't work for so long, especially a Jennings. Anyway, I traded the hollow-point 9mm ammo in his magazine with ball rounds and it worked perfectly. His mouth just dropped open in amazement. He claimed it was the first time it had ever fired.

No, I've learned that just because they own it, sometimes for a long time, it doesn't mean they know anything about it.

Post your question on the Shotgun World gunsmith forum. You are sure to get a good answer there.

http://www.shotgunworld.com/bbs/

I wonder if the barrels are actualy firing simultaneously, or in very rapid succession. If held loosely, the gun can recoil enough to reset the trigger and then rebound forward,in effect pulling the trigger a second time and tripping the sear for the second barrel. I used to encounter this occasionally on the skeet range, and invariably the cause was the shooter.

I am no expert - I have had 2 of these guns here in NZ & I still have one as my normal shotgun. While do not use it a lot, I have hunted ducks with it and sporting clays.

I have never faced the problem you describe.

I have deliberately taken 2 quick shots which sound almost like a roll and had no problems.

You're body can do strange things . I had badly sprained my wrist .During the months of recovery my BHP doubled !! I had been doing work on it and every time I shot it it doubled.One I had a friend along .Gave him the gun but evry shot was without doubling even though he wasn't a pistol shooter it never doubled. For a while after full use of my wrist it would occasionally double . But only the BHP no other gun !! Finally it worked OK.

I barrowed a Garand for a match it double on me several times. Needless to say my score sucked owner said he never had any trouble

Beats me sounds to me the owner is bump firing it some how.