Step 1 – Install the plugin called “Yoast Duplicate Post”. 
Go to WP-Admin > Appearance > Install Plugins > Add New – Search for “Yoast Duplicate Post”, install and activate it.

Step 2 – Select Custom Posts you need to duplicate.
Go to WP-Admin > Setting > Duplicate Post – Select the “Permissions” tab. Check all custom posts wanted

Step 3 – A “clone” option is now available below each of your posts.