Login into your SignMyEmails account & hover over your name or the drop down option in the top right corner to view options. 

Go to the 'Dashboard' & click on the 'Duplicate' icon for the respective signature(s) in the 'Actions' menu to duplicate the signature.

Alternatively, you can duplicate the signature from the Signature Install popup too. On the Signature Install popup, use the 'Duplicate' button at the bottom of the popup to create a copy of the signature.