I had an old J Stevens years ago which did the same thing. Different model but could be the same condition, worth checking. The left sear sat slightly lower than the right one. The sears were slightly loose on the pin. They would on occasion make contact & if the left triger was pulled first, both sears would lift, giving virtually a simultaneous discharge. I put a small coil spring (ball pint pen as I recall) on the pin between the sears which kept them pushed apart & it never occured again. Definitely check all the things Jim recommended as well, they are all possibilities.
