Duplicating an email can be useful if you are wanting to just changing the content without changing the layout. Another method to achieve the same goal is to create a master template.
To create a duplicate email, go to campaign > emails.
When you are on the emails screen find the email you would like to duplicate and click the orange action button.
When the action button is clicked you want to click duplicate. The email will then duplicate with ‘copy of’ at the beginning of the email name.