Hey Guys, First time user. I need some advise. I have a Glock 21 which is one of the older model ( approx. 10 years old.). I haven't fire it much until about a year ago when I joined an indoor range here in Calgary Alberta. After about 500 rounds it started to double fire every once in while. One of the range owners said this was a common problem with the older glocks and he would change some trigger parts and fix. it. Well it worked for about the 1,000 rounds now the problem is back again. I took it back to him and he said he wasn't sure what he did to fix it in the first place so I don't really want him too look at it again. Can someone tell me what the problem is and how it can be fixed? Proud owner of a Glock 21 and 23.