Duplicating menu items may happen on purpose or by accident.
If you accidentally create a page with the same title as a page that already exists and you allow the Automatic URL Alias to be created, Drupal will add a version number, a dash followed by a number (-#), to the end of the path. The first question, is really why would you need two pages with the same title? It’s a good idea to merge the pages, delete the duplicate and remove the version number from the path by using a Custom UTL Alias.
There are good reasons to want to put a page in two different areas of the website navigation.
The Drupal menu system requires a page to “live” in a specific place in the navigation. It will have a specific parent, unless it’s a top-level navigation item, and if it has sub-navigation items, they are its’ specific child navigation items.
When you try to place a page in 2 places in the navigation, sometimes Drupal won’t cooperate. No matter what you do, how many times you set and re-set the menu items, for some reason it will not display in the right hierarchy.
Solution: For the additional menu item, the one you want to take the visitor to the other area of the site, add it with an path that is the absolute url in the Main Menu Navigation interface. Absolute paths, include the the “https://” and root part of the address.
Normally, we want to avoid absolute paths to other pages within the same site. On the test version of the site these will link to the live site. We have actually thought we were on the test site and done some test work because we clicked on a path that took us to the live site and we didn’t realize it. Also, if the root url for the site ever changes this link will be broken.