Unfortunately SMF is not that powerfull. It normally just gives you the ability to thumbs up or down a post and it doesn't even keep track of where it was. I had to hack in the functionality so we can see where a karma was given. So anything like an undo is not possible. The ability to track down where it was given to prevent giving another one is not possible either.
Adding that you can see which ones you gave is, but since I have to add this to the script and my time is limited, I need a good reason.