Upon firing, my 10 mm Glock 20 will often double or triple (sometimes more) fire upon the initial squeeze. I've put many, many thousands of rounds through this piece since 1990 and I am don't think it's me (ex Naval Gunner so I think I know what I'm doing). Some shops say "so what's broke about that?", and "do you want to sell it?". No! I do not want to sell it and I am able to control the multi fire well at even 50 yards, but the ranges won't let me shoot it. I've heard that it could be a crack in the receiver but I cannot detect one. Anyone heard of such a thing? Suggestions?