If, as CareyH asks, it's actually firing one when the trigger is pulled, and again when the trigger is released, the disconnector needs to be repaired or replaced.

To check for a worn disconnector, with the firearm unloaded, hold the trigger down and pull the charging handle to cock the hammer. Then, VERY slowly, let off the trigger. If the hammer falls instead of dropping off the disconnector onto the trigger sear, your disconnector is the problem.