I'm using PHP5 and I have a directory where people can submit websites.

Is there a simple built in phpMyAdmin setting to block duplicate submissions?

So, quite simply all that I need to do is have the URL row checked for duplicates and if the URL already exists in the db block the duplicate submission altogether.

I've of course Googled this, however the only promising how-to's assume that you have a lot of back-end knowledge already, which I do not.. I'm a front-end guy.