I have been having problems with keyboards on my new Windows 8.1 build. I have been able to replicate this using 2 different USB keyboards (Logitech G15 and Razer BlackWidow).
Every once in a while when I am typing, a key will get "stuck" and type out several times on the screen. So for example, if I was typing the word update, it might look like this.
At first I thought it was the keyboard, but since I have switched and just had it happen on the new keyboard.
I have switched USB ports on my mobo and that didnt seem to help it.
I am using the default Windows drivers as well for the keyboard.