You can create a new email message from a previously sent message. This will create a new email and a new sent message report. There are two ways to do this: From the Email Composer, or from your Sent Messages.
From the Email Composer
Click Outbound > New Message > select Sent Message
From Sent Messages
You can also go to Outbound > Sent Messages and hover over a previously-sent email message, and click New to begin using the email composer.