I bought a new, 223 AR-type rifle recently at a gun show. It has a Plum Crazy lower, and a Wilson upper. I just got around to sighting it in today. After shooting 20 or so rounds, the rifle started shooting two shots each time I pulled the trigger. I held the trigger down to see if it would shoot more than two, but it didn't. It's actually kinda neat, but I need to get it corrected. Is this a common problem, what is the cause and solution?

Thanks,

Jack