Duplicate Post is the most popular plugin for duplicating posts, with over 3 million downloads.
Once you’ve activated it, it adds a Clone link to each post in the main Posts screen in the WordPress admin.
Click the Clone link under the post you want to duplicate, and a new post will be created for you with the same content and title and a Draft status.
You can then edit that new post and give it a new title.
It also adds the same Clone link to pages and for custom post types (if you enable them in the settings).