While I tend to like email notification and there is something to be said for it bringing people back to the site, since email notification is not pervasive through the SE platform we should not work with the assumption that it is enabled.
I asked about this in today's moderator chat session. This was the response I received:
assuming the user a) has an email b) has opted in to email FOR THAT QUESTION.. so it'd still be quite rare
remember, email is 100% not required to use any of our sites, and we never send emails to anyone unless they ask for it on a per-question basis
I think leaving a comment is sufficient, since that will trigger the site notifications. adding an answer is arguably wrong since it's not really an answer..
we generally work under the assumption that users are coming back and looking at their questions.. the email option is really just "last resort" for users who can't or won't.
Original transcript (moderator only)
So, I think we should just leave a comment pointing to the canonical question for the duplicate(s) and leave it at that.