The help center tells us the dupehammer works with the original tags of the question:
A question can be closed as duplicate with a single vote from a moderator or a user who has earned a gold tag badge in one of that question's original tags.
That, however, is no longer the case:
First, we now look at the question's current tags instead of its original tags, unless you were the one that edited them (either by doing it yourself, or by approving an edit from someone else).
Please update the help center to reflect the change.