Should I mark as duplicate if the question is answered elsewhere, but it's not the same question?

Question

I'm looking for policy or guidance on the following situation. Someone just posted a new question Q1. I noticed that one of the existing answers to another question (Q2) already answers Q1 -- the answer to Q2 happens to be general enough that it answers not just Q2 but also Q1. However, Q1 is not the same question as Q2: they are somewhat related, but they're definitely different questions. In particular, not every good answer to Q2 would answer Q1.

What should I do in this situation? Should I mark Q1 as a duplicate of Q2? Should I avoid marking it as a duplicate? Is it a judgement call? Is there any general policy or guideline?

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Where I looked for information on this

The closest related question I could find on MSO was At what point does a question become a duplicate?. There, Shog writes "If a new question is answered by the answers to an old question, I will generally consider it a duplicate. The exception to this is when an answer to the new one does, or potentially could add a significant amount of useful information." So that makes me think it is often OK to mark Q1 as a duplicate of Q2, but not always.

The official guidance says to close a question as a duplicate when it is "sufficiently similar to existing questions and would be answered identically to them", which makes it sound pretty simple: do not mark Q1 as a duplicate of Q2.

If it's relevant, when a question is closed as a duplicate, it is marked with the label "This question already has an answer here:". I suppose this could be taken to suggest that it's OK to mark Q1 as a duplicate of Q2, though personally I doubt that we should be looking to that wording for guidance or policy about duplicates.

I've read Close reason proposal: "Answer exists elsewhere", but that sounds like a different situation to me, so I'm not sure whether the discussion there is applicable. (For those who read Grace Note's answer there: For instance, the mkdir example is not quite the same situation, because those were two unrelated questions, and the hypothesized answer is not likely to be a good answer to both questions.) I've also read Should we answer questions if the answers can be found elsewhere on the site?, but I'm not 100% clear on how that applies to the situation I gave above.

If you want to see the specific situation where I ran into this most recently, it is this question: Decide whether a DFA accepts the empty language. But I most interested in the general policy/guidance for what to do in this situation, as I see this sort of situation from time to time.

Answer 1

If the questions are different, they shouldn't be marked as duplicates.

The solution here is to comment (or post an answer) with a link to Q2's more-general answer. If you choose to post an answer, it ought to include some of the general answer's content as well (not merely a link). Note that this answer may not solve Q1's problem exactly, but it provides some food for thought.

Q2's general answer then shows up where it otherwise might not be seen, allowing:

1. Q1's viewers some insight into a related question's discussion
2. Q1 to receive answers tailored to Q1. If marked as a duplicate, Q1 loses potential good answers, since "not every good answer to Q2 would answer Q1."

Answer 2

In general, questions should be closed based on being a duplicate of another question. However, in the cases of proper subsets, it's ok to close a question as a duplicate of a more general question if the answer to the general would answer the specific. You really have to decide on a case-by-case basis on these.

An example where it doesn't work well is something we see a lot of on Meta, where a specific tagging question is asked and attempted to be closed of a generic tagging question, but the specific one has to have a specific answer, like burninate or not burninate.