50405 ban url / stop double adding

Hi, I run an autosurf and I am running into problems where users keep resubmitting the same site page over and over again so I would like the following When the user adds sites from there members admin panel I would like the URL to be verified by looking into the sites table in the database to see if the URL has been added. So for example if I go to add [login to view URL] and it is already in the database a message will return to the user telling them the URL is already added and to please submit another page of there website. If the url isn't there then it will be added with no problem.