If a user votes to close a question as a duplicate, and that user doesn't have a dupehammer that can close the question right away, the user who posted it has the possibility of accepting that the answer is a duplicate. And that acceptance becomes visible, besides the fact that then the question gets immediately closed.
I suggest that an acceptance button could also exist when a question gets closed by someone with a dupehammer. In that case, its only effect it would have would be to make visible the fact that the person who posted the question agrees that it is a duplicate. But that's much simpler than having to write a comment stating that, and it would provide feedback to the owner of the dupehammer.