I had a staple stuck under a key one time and did not realize it, the letter would type multiple times. I finally picked it up and shook it in anger and I heard the staple!
I unplugged my keyboard and tested it out on another computer, where it worked fine. So I brought it back to mine, and now it works fine. It's a wireless keyboard, and it's the transmitter/receiver thing that I unplugged, so I wonder if it just needed some sort of reset (shutting down the computer didn't work).
Eh, just FYI, in case anyone else runs into this sort of thing.