Unique Field module for Drupal

The Unique Field module for Drupal adds the useful option to require that a given field have unique content. Without this module, Drupal does not prevent multiple content nodes from having the same title or same value within a given field. This module performs additional validation when a content node is created or updated, and prevents users from saving a node until the field is given a unique value.

@ Unique Field project page on drupal.org