Hi Ronnier,
Edited-
What exactly you mean by the duplicate document? Are you looking for saving a document's copy so that you can make further changes later?
There isn't any option to duplicate the pages in Fill & Sign. As you have mentioned that your boss has been able to do that, then there is a possibility that he has been creating a duplicate copy of the document by choosing "Save As" option.
Also, are you referring to the desktop application Adobe Reader DC's "Fill & Sign" feature or Fill and Sign mobile app? Because Save As option would only be available with the desktop app.
We need more information to understand your requirements in feature.
Regards,
Akanchha