I've seen that on a number of tang safety guns, both singles and doubles. Generally just a problem of tinkering with the fit of the safety and its actuating mechanism. Can also be caused by trigger return spring problems or just a bad need of lube. One thing right off to try is (unloaded) cock, push the safety off then push forward on the back of both triggers and try to set the safety manually. If this works, it just an actuating movement or spring problem.