The Karma (reputation) system operates on an AJAX system that makes a background web request whenever you press the button. Due to the way the database queries are executed when a thread loads, we don't query which users have already given reputation to which users. When Revenue Source gets to be a large site, doing so would be incredibly taxing on the servers considering it's one new database query per post in a thread.

I completely agree that it can be annoying going to give reputation to someone and then getting the "You can't give reputation to the same user twice" error. Myself and another database engineer are looking at the system to see if we can't work around the above limitations.

We're also investigating if it's possible to hide the button in instances where you can't give reputation to the user (due to permissions). I'll post more here when I get an answer.

Thanks for the good suggestions, Light.