Prevent duplicate link submission

Is it possible to prevent user to submit a unique Url more then one time?

I want to build a node type with cck where user can submit a story and attach a surce link. Now, when another user wants to ad a story and ads the same url, he should get an alert his story is already in the system. For usability a link to the existing node would be nice.

So, when saving the Url field, there must be a check if the unique url is already stored.