Closed and reopened questions for reasons of being exact duplicates continue to carry the "possible duplicate" box. Is it polite to edit the question and remove the box. I fear I might not get answers as people might assume duplication?
I agree with Arturo that the box should be removed. I would add: If there aren't too many comments under the question, then it might be helpful to add a comment explaining why the question is not a duplicate of that other question. Alternatively, there are cases where it's good to add that piece of information to the question.