My first double was an old J Stevens Arms & Tool Co 12 gauge. It was a 2-trigger gun & doubled on me once when I pulled the back trigger first. An investigation revealed the holes through the sears for the axle were worn & the sears could rock a bit. The left sear sat just a bit lower than the right one so if they happened to get together & the back trigger was pulled it would lift both sears. There was space between the sear bodies so I put a small coil spring around the axle to keep the sears pushed apart & it never doubled again as long as I had it.
There are enough different possibilities as to the cause that a Hands-On diagnosis is essential for proper diagnosis. In the case of my old J Stevens as long as the right trigger was pulled first it was not truly a safety issue, more of an inconvenience. "IF" it is from a worn sear tip or bent in the tumbler this is a definite safety hazard. Pulling the other trigger first is not the answer in this case as any abnormal Jar could set it off, & it might not have to be all that abnormal. It should definitely be tended to.
