How to duplicate existing page
To duplicate an existing page, please use a great free plugin: Wordpress Post Duplicator
After you install and activate the plugin (in WP Admin > Plugins > Post Duplicator > Activate), go to WP Admin > Pages
Locate the page you want to duplicate and hover a mouse on it.
You'll see a new option appeared - Duplicate Page. Press it to duplicate.
A new page will be available immediately.
To learn how to add the new page to the site Menu, please see this instruction.