What should be considered ‘duplicate’

I have seen questions marked as duplicate, which could be considered so only if/when the answer is known. A question I noticed this about is here. Should these be considered duplicate? Obviously the OP asked it because he had no clue his issue was related to missing Access-Control-Expose-Headers. Similarly, Googlers finding the Q&A latter will also have no clue that is the issue. Isn’t the question legit/useful then?