Currently, it isn't possible to duplicate campaigns. However, it is possible to resend letters previously sent from a campaign.
To resend letters from a campaign:
Access 'Letters' at the top bar.
Click 'Campaigns' at the left side bar.
Look for the campaign you wish to resend and click 'View' on the far right.
Select the letters you wish to resend and click the 'Resend letters' at the top or 'Add to a campaign' if you wish to create a new campaign.
Choose or create a template to your new letter.
Edit the recipient address and recipient details, if necessary.
Select the letter line, click 'Add to mailshot' and then click 'Go to next step' at the button right corner.
At the Summary, choose one of the send options, a name for your mailshot and click the green button at the bottom right corner to complete the process.