Avoid duplicated external links I would like to add a section on my website (Drupal 7) which consists of articles with external links (as source). To avoid spam and duplicated content, I would like to avoid that users may add a URL more than one time. Basically, I need something which checks if the external URL is unique and not being used more than once.

I've looked for modules on Drupal.org, and tried googling some information. But I couldn't really find a solution. The Link checker module seems fine for checking if the URL is still available, but it doesn't avoid the same URL is used more than once.
You should take a look at the Unique Field module:


  This module performs additional validation when a node is created or
  updated by a user to require that a node's title, author, language,
  and CCK fields are unique within a given context.