I have had a Parker Bros. gun double fire before and so has a friend. In both cases it was a worn sear. Larry Delgrego & Sons in Ilion NY can repair it. They have the original Parker machine tool that will restore the proper angle/edge on the original sears. It is a quick process. Continued doubling risks cracking your stock. Additionally a weak sear can allow a gun to fire if it is jarred, even with the safety on. Best to get it fixed.