On a few forums you can only rep the same user once like here if u tryed re=repping it would ask to update on mine you can rep as many times as you like anyway to solve this to stop rep abuse
Go to ACP > configuration > reputation and disable multiple reputation.
So you would like to limit it on your own forum as well...?
If so go to Admin CP > Configuration > Settings > Reputation > Allow Multiple Reputation > No.
Grazi grazi problem solved