This article explains more about the different options when folders need to be duplicated.
Folders cannot be duplicated, but it is possible to duplicate pages and whole workspaces.
In other words, if you are in need of duplicating a specific folder, you might want to consider duplicating it one page at a time and then creating a new folder to which those duplicated pages can be put under.
If you'd like to duplicate a huge number of folders, you can consider the option to duplicate a whole workspace instead, and then delete those pages and folders that are not needed.
You will find instructions on both page and workspace duplication from Help center.