Are you using a Hotrod by chance? This has been a recurring problem in my upright cab. Go to notepad and see if one flick of the joy gives you multiple key presses, which it does for me. There might be something weird going on with the ps/2 pass thru keyboard if you have that kind of setup.
In my case, it doesn't seem to matter during games, but in the mala menus and to set up key assignments in some emulators the multiple key registers is a huge pain! I've tried to mess with windows properties about keyboard repeating with no luck.
Please do post if you solve this problem, as it might be helpful.