There can of course be a number of things. Is it 1 or 2 triggered, you haven't stated this as yet. Does it always do this when firing the same bbl first. I once had an old J Stevens which would occasionally double if you pulled the rear trigger first, but it never did it when firing normal order of front first. Turned out the pin holes were worn in the sears & the left one set slightly lower. If the tail ends over the trigger blades happened to get together pulling the rear trigger would lift both sears. Was a pretty simple fix. When you get the stock off with everything else still or re assembled you just have to look for anything which doesn't appear right. May be easy to find, but may take a lot of scrutiny.
Of course as mentioned it may not be the "Gun" doing it.
