it seems nobody got the real source yet:
'Ditto' comes from Latin, and means about 'as has been said before'. So most of the other explanations are correct.
there's a lot of examples around already.
Wicked Witch of the West.
'Nickname' for an evil (female) person, named after the famous Wizard of Oz movie.
the WWW called again, she wants us to come over for Thanksgiving. No way, if you ask me.