This is called key 'chattering' or bouncing, Google will give you a better definition of what this is but basically, this is what is causing your problems.
I have this on my old Noppoo choc mini and was unfixable, but there are a few steps you can take to make it easier or if you're lucky maybe even fix it?
Remove every key and clean (thoroughly) the entire keyboard, remove any dirt, hairs etc, make sure it is entirely spotless.
Other than that you may have to unsolder and re-solder a new key for that switch, but that's more along the lines of an advanced user and I wouldn't advise that if you are not comfortable with doing so.
The only other option is to obviously.. buy a new keyboard.
