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.
You can contribute to the development of this plugin on GitHub.
Help translate this plugin into your language on translate.wordpress.org.
Screenshots
A post with a unique title using the "Classic Editor"
A new post with a duplicate title using the "Classic Editor"
A post with a unique title using the "Block Editor"
A new post with a duplicate title using the "Block Editor"