In a couple of recent situations I have found myself at odds with the duplicate question policy of the site. The most recent case was this question Proof of Ohm's Law [duplicate] that was marked as a duplicate of two other questions which not only were asking different things but also none of the answers really addressed the OP's question. I actually found a good answer for the question in this arXiv article but I could not answer because the question was already marked as duplicate (I would need to read the article very carefully because I find it a bit complicated).
What I want to note with this is that the duplicate policy (as it is now) has several flaws that reduce the effectivity of the site as a place to share knowledge:
- Some questions that ask different things are marked as duplicates if the prior question has an answer that may vaguely be related to the latter question. This is clearly a problem because when one answers a question, if you do it right, you have to take care to address the specifics of the question. Redirecting a question to one that is only somewhat related will very likely undermine the nuances of it, and it may not even be a proper answer for the question.
Physics evolves and hundreds of articles are published every day. For example if I ask about cosmic censorship in an unspecific way, the question would be immediately marked as duplicate. No one would be able to mention the existence of the $C^{0}$ stability idea, which is a rather new topic. The result is that even if there is new physics and new ideas, these cannot be shared as effectively because something related has been asked before.
There is no specific set of guidelines that render a question a duplicate. This makes the decision of marking a question as duplicate a personal decision of the mod, if you are unlucky enough to find an intransigent mod (which are a thing in PSE) before you get a good answer then you will never get an answer. The question boils down to: What constitutes a duplicate question? Is it an exact & paste? Is it asking the same thing but with other words? Is it asking for something previously answered but with a different focus? Is it asking for something already answered but in more detail? Is it asking for something that has not been asked before but someone already answered in another question? Is it asking for something related to an answer to an existing question? ─The problem is that for me only the first two would be duplicates whereas for some users, all of these consititute duplicates.
- There is no formal mechanism to appeal to a duplicate question. You can flag, delete, edit and report questions with a single button, but there is no real way to appeal if your question (or one you like) gets marked as duplicate, unless you are a mod or have the gold badge, which most users are not and do not.
I think the solution could be to make a more formal process of appeal (resembling the review queues) where the user is allowed to make a case for his question if he wants. And should the appeal be rejected, a good reason should be given (at least a better reason than "it's duplicate because I say so" which is what is basically implied when the only comment is "possible duplicate URL", and any debate is dismissed or ignored).