If someone else has a gold tag badge that works for that question, they can always override your duplicate decision with the click of a button, whether you also had the badge or not and regardless of your status as a moderator. That's what they're trusted to do - single-handedly close and/or reopen a question using that tag.
However, as a moderator, you do still have slightly more power than they do. You have the ability to close the question again if you truly feel the need to do so. As a regular user though, they are still limited to only casting one reopen vote in the lifetime of that question. They wouldn't be able to reopen it on their own again. The dupehammer may be a useful and powerful tool, but you can only wield it once per question (well, twice since technically you could use it for both a close and a reopen).
As far as a user being able to use the dupehammer on their own question, it has been discussed before but it's not seen as a pressing problem at the moment.