To create a copy of a Gliffy diagram:

Copy the whole wiki page, and then change the WML code to point the Gliffy reference to the current page's attachment. Copying wiki page also copies attachments, but the new duplicate page still refers to the old attachment (and that reference is now invalid since it's not a part of the current page). So by changing the WML code to refer to new page's attachment all is well and the original document and Gliffy are not effected.