Module for checking submitted url duplicacy!!

I am trying to make a website similar to digg but without using drigg module. I have one option of using Pligg cms and another option is using drupal. But D6 drigg is not available, so i am trying to make it using drupal6 with five star vote module. I would like to know is there a module which checks whether the link being submitted is not already submitted by someone else. If not? from where should i start to write my own module for this purpose.
can this module be used as a reference to check whether the url being submitted is already submitted by someone else??
http://drupal.org/project/username_check

Thanks!!