Unfortunately, OneNote  Class Notebooks cannot be duplicated. However, you can make a new class notebook and Copy an Old Content Library to it.
 

1. Create a new notebook for the class that you are transferring content to. The process for doing this can be found Here.
    
2. Ensure that both the Old Class Notebook (from which you are copying) and the New Class Notebook (that you are copying to) are in your OneNote  app.
   
    
3. In the Old Notebook, navigate to the Class Notebook pane and select Copy Content Library.
   
    
4. On the Left hand side of the panel that appears (picture below), select the Sections of the notebook that you wish to Copy (this includes the welcome page, and pages from the Collaboration Space and the Content Library).
   
    
5. On the Right hand side of the panel, select the Notebook(s) that you wish to copy these sections to and Press Copy.
   
    
6. These sections of the old notebook will now appear in the new notebook. Please note that these duplicate pages are independent, so Editing In One Notebook Will Not Affect The Other Notebook.