Search for duplicate or similar node titles when creating content?
There is module Uniqueness , it does exactly what you want. It allows you to enable for any content type, and you can configure checking on node add or edit or both.


  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.