For whatever reason, it sure appears you're not getting complete trigger return. Weak rebound spring? Has the rebound spring been cut? Whatever the cause, I suggest replacing the existing rebound spring with the next pound rating, i.e., a 14# if you currently have a 13# in place. See if that cures your double action trouble. If not, try a 15# rebound spring.
That's the only thing I can think would be causing your trouble, based on your description. I hope this was helpful.
