Yeah, that's a problem, alright. It's known as "hammer follow."
Two things: Do you know the age and condition of the springs in the gun? If it's used (I think your model was replaced with the Mustang, so I'm assuming it is), you should order up a full spring set from Wolff. I never trust somebody else's springs. A bad sear spring will often cause hammer follow.
Second, can you replicate the problem on the ground? Unload, lock the slide, and then let it ram home. Don't do it more than once or twice, but see if the hammer follows. Repeat with the trigger held down. See if the follow stops. If it does, see above commentary on sear springs.
Beyond that, I got no idea. If springs don't fix it, it's probably a disconnector/sear/hammer issue, and I don't know how your disconnector works. I know enough about the Mustang to be real dangerous, but that's about it.
