Removed SuperDuper from Growl's list, it re-added itself on next run. Changed notification type to mailme (individually for all 4 notification types) and the problem remains.
Hang on... wait... the problem is in Mac OSX Mail: full text of email is:
Subject: SuperDuper! Copy Succeeded
User-Agent: Growl/1.2 simple-mailer/1.0 python-smtplib/2.6.1
Copy of KingstonSSD to Clone KingstonSSD was successful.
-- This message was automatically generated by MailMe, a Growl plug-in, --
-- in response to a Growl notification --
--
http://growl.info/ --
OK Dave, problem is probably not yours!