Duplicating a page
To make a copy of a selected page in the Book Editor, hover over the page to select it, then click on the Duplicate icon over the page thumbnail. A copy of the page will be created.
To make a copy of a page in the Page editor, click on either Copy icon on sides of the page. A copy of the page will be created on the selected side - either before or after the current page. While it won't appear that anything changed, you will be focused on the copy of the page - Memento Pro will let you know the duplication was successful.
It is not (yet) possible to duplicate or save an individual a page as a template or to add it to another project.
Deleting a page
To delete a selected page in the Book editor, hover over the page. Click on the delete icon over the page thumbnail. The page will disappear. Use Ctrl-Z (Windows) or Cmd-Z (Mac) to undo the deletion if required.