To duplicate a newsletter, simply click the Duplicate button in the top-right corner of the newsletter's detail page.
You will then be asked to enter a new title, subject, and preview text.
This action is meant to duplicate the content to be used for a new newsletter. It's a great way to essentially have a custom template. All content will be copied over but Recipients and Discount settings will return to their default values of All Subscribers and No Discount, respectively.