The question is simply how many downvotes a duplicate question that was posed most likely without any ill intention deserves?
The closing as duplicate is the important thing and the answer with additional comments is a very good answer, although I feel it should be optional. A comment explaining a bit why the cause and the remedy are the same would have been enough.
What remains are 7seven downvotes that serve no direct purpose.
The question itself is fine it's. Its only realyreal problem is that it is a duplicate. How much research was done before? Could the duplicates have been found easily before? I don't know.
Please note that even such questions have a value for SOStack Overflow, because they represent additonal search terms to come to the same solution.
My idea would be to stop displaying votes and stop voting on questions that are duplicates. It's mostly meaningless. A simple fixed penalty for posing a duplicate question (could happen to the best of us), from 0 to -10 repsreputation points maybe, should be sufficient. My feeling is that downvotes and closing votes are somehow too much of good things at the same time.
As for rudeness: it's for everyone or for nooneno one. If we want a certain level of courtesy we should insist on it everywhere.
The best is to do what you think should be done but also to move on and take a break before bad feelings occur.