Is there anyway of checking for duplicate links?

Originally uniqueness was enforced by the database, with the URL field set to a unique index but people wanted to be able to copy entries to other categories, so that index was removed from later versions. In any version there are checks if a site is already submitted, awaiting approval it can not be submitted again.

If you are familiar with PHPMyAdmin you could check, navigate to the database that your installation uses, find the links table and look at the structure of the table. You could add a unique index to the URL column if it doesn't have one already.