How to duplicate existing page
To duplicate an existing page, you can use a dedicated plugin for this job: Wordpress Post Duplicator
After you install and activate the plugin (in WP Admin > Plugins ), 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.