Prevent duplicate link submission when using CCK Link module with AJAX alerts
Searching the link module issue queue for the text "duplicate" brought about a few others working on this feature.
In response to a 4.7.x era feature request How to avoid duplicate entries for a link field?, quicksketch points to hook_form_alter function and http://www.whatwoulddrupaldo.org/catching-duplicates-with-hook-form-alter
The issue came back and a patch was even provided for Drupal 5 in Allow only unique URLs.
Currently, the Unique Field module has a solution, but requires some changes from the link module: Using the Link and Unique Field modules.