This plugin checks the title of a new post/page or any other post type for uniqueness. The plugin provides a filter unique_title_checker_arguments, which enables you to modify the WP_Query arguments used to find duplicate titles. You may use it to search in more than only current post type for a duplicate title.
This plugin is an enhancement of the Duplicate Title Checker by ketanajani which only supports posts but not pages or custom post types.
