Should be bugzilla:duplicate ('exists' is for 'is already live on site and i missed it somehow') but don't worry about tagging it that way -- adding any of the bugzilla:* tags can make my count look off, because I also have to tag the admin entry that goes with the suggestions entry! (Every bugzilla:* tag has a corresponding admin:* tag that goes on the admin-only entry that goes with each suggestions post; it's a handy cross-check to make sure I don't lose track of anything.)

(Don't worry about it in this case, though; I'll go fix this one's admin entry up right now.)

BTW: unless there's any significant discussion on the suggestion post before someone called "duplicate", you don't need to do the various site: and workflow: tags for a duplicate suggestion -- I don't delete duplicate suggestions if I accidentally let them through the queue, but we all pretend we can't see 'em ;)