The only thing restraining it from playing the second 'ghost' note is the weak spring inside the switch itself.
The problem is this: the strum bar sits right on top of two switches, one for the up strum and one for down strum. Once you get into hard mode, it's a near-impossibility to get through without strumming up and down, and passages that are played perfectly will get you booed off the stage. The harder you strum, the harder it bounces back: it's especially bad on the up-strum. When you strum the bar, it will play the note, but then spring back and play a second note on the rebound. Those of us in the last two category have had to fight an enemy most foul: the double strum. Still others use an actual pick (you know, for heightened realism) to play the strum bar. Some (like myself) actually strum the 'strum' bar', brushing it with my fingernail as though I were picking an actual guitar string. Some players grasp the bar between the thumb and forefinger like a pick, rocking it up and down to play the 'notes'. There's the 'thumb' technique, where the player pushes down on the strum bar with his/her thumb to play a note. There are a few different techniques for playing the guitar hero controller.
