Copying a menu can be a useful tool for many scenarios. For example, if you decide you are going to have a weekend beer menu and the menu will only change slightly, you can create a copy of the main menu to use as a base.
To start, check the box to the left of the name of the menu you'd like to copy:
Click Duplicate:
You will have the option to rename the menu (the default name will be "Copy Of -menu name-") as well as choose whether the menu should go directly to Published status or not:
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You will now see the new menu on your menu management page where you can work with it as normal.