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.