Lately, I've seen a number of questions closed as duplicates with only gnat's vote, such as When to make classes that do multiple things?
As far as I know, the only legitimate way to do this is by having a gold badge in one of the associated tags. As gnat's one and only tag badge is a bronze badge in java, I can only assume this is a bug.
It's not single-handed: notice the presence of Community in the close voter list.
Last year a feature was introduced to Stack Exchange to allow question askers to single-handedly close their questions as duplicates.
When someone else proposes a possible duplicate, the original poster (and only that person) sees a banner at the top of the post:
If the poster clicks the "That solved my problem!" button, the question is closed as a duplicate. The Community "user" is given the responsibility of being the final close voter, but any other voters are listed in the close banner as usual.
So what's happening here is that gnat has proposed a duplicate, and the question asker agreed that the proposal solved the problem.
If you mouse over the "Community" user on that post, you will see the following text:
This question's author approved a pending duplicate vote.
Also note that gold tag "dupe hammers" look different (this is from SO, not sure if we even have any gold tag badges here):
The question you linked has "Community" as the final vote, not a regular user with the gold tag listed.
It's not a bug. There's a feature where, when a question is voted as a duplicate, the asker is presented with the other question and prompted to either confirm that it's a duplicate or to review the other question(s) and edit to explain what makes their problem different. This is a relatively new feature, but you can probably find more details on Meta Stack Exchange.
