Kai here from Shopify, thank you for your question in regard to the menu in your Shopify Shop.
You would usually duplicate menu items in the admin under Online Shop > Navigation > Main Menu.
The whole process is described in detail in this help doc.
If you are using a menu app this process might be different. If you are using an app to enhance your menus the developers off the app would be the best place to ask for support. You can find their FAQs and contact information on the app page in our app store:
Please get in touch if I can help with anything else.
Hope you'll have a great day,