Uniqueness module provides a way to avoid duplicate content on your site by informing a user about similar or related content during creation of a new post. A UI widget is added to the node/add form that does asynchronous searches on inputted fields (like the node title or vocabularies) and returns the titles of similar content.
The module supports three different search modes for finding similar content: Simple node title search, Drupal search, and Apache Solr.
Security note (6.x only): if your site relies on node access permissions, you should use either Drupal search or Apache Solr, because Simple node title search does not respect any content access settings and as a result may display protected content. (See #1119656: Node Access Check for "simple node title search").