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.
All those are configurable options.
Search words:
drupal prevent duplicate submissions
drupal prevent duplicate node titles
drupal deny duplicate content
I was getting all excited about writing this module when I finally found it, at the last second, while brainstorming titles for it. (No, unique field
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