Straight from the FAQ:
II. Marker Won’t Recock
A. The gun is cycling (stuttering or making a loud BRRAAAAP noise like it is out of air) or marker shoots twice or more when the trigger has only been pulled once.
First, make sure it is not out of air.
Hammer O-ring – A seriously worn or missing Piranha hammer o-ring (35) will cause the gun to cycle uncontrollably. Do not replace with a bottle o-ring! A bottle o-ring on the hammer will also cause the gun to cycle like it is full auto. A old Piranha hammer o-ring is clear in color, like a bottle o-ring, but much softer. Newer Piranha hammer o-rings are blue. Hammer o-rings do not usually go bad unless you are not oiling the gun or not using synthetic gun oil.
Sear Pin - A sear pin that is coming out of the marker will cause the gun to cycle. Lightly tap the pin back into place with a hammer. If the problem continues, remove the pin and re-instal it on the other side of the marker.
Sear – A rounded, chipped or broken sear could cause the gun to cycle. A worn sear will cycle increasingly as the sear wears out more and more. A truly worn sear will cycle until the cocking rod is stopped or the tank is out of air. Inspect by taking the grip frame off the receiver and looking at the back tip of the sear. If it needs to be replaced, knock the pin out of the grip frame from the left-hand side so it slides out the right and replace the sear.
Hammer – If the lip of the hammer (14) that catches the sear has mushroomed up or been bent, it will not catch the sear properly and the gun will cycle uncontrollably. Replace if that edge is deformed. There is an anti-rust coating on the hammer that will scratch off where the sear touches it. This is part of normal wear. You're looking to see if sear has actually deformed the lip of the hammer.
Regulator too Low - If your regulator (tank or vertical regulator) is set too low, then your velocity will be low and the marker will not recock properly. Raise the pressure.