You can duplicate most messages using the "Save as..." option but old or unsupported messages aren't duplicatable.
Old HTML code that is no longer compatible with the editor:
You need to start a new message by clicking the "New" button on the list of messages page or duplicate another, newer email.
Old dynamic content {@insert-X} tags:
In this case you should replace the {@insert-X} tags with dynamic content using the new dynamic content editor.