We close Question A as a duplicate of Question B only when all of the following is true:
- It's the same question, or Question A is already covered obviously as a subset of Question B. Obvious here means I can tell at a glance it's there.
- Question B has an obvious answer to Question A. Obvious here means I get a straightforward answer without hard searching — a couple of sentences buried in the middle of a post, or an answer which only sort of implies an answer to Question A, doesn't count as obvious.
- There is not some strong compelling reason to covering Question A alone, separately from Question B. (If the above bullet points are met this rarely happens.)
The idea here is it's the same question with an obvious answer. Someone redirected from Question A to B can see obviously why they were redirected and find a satisfactory answer.
I've seen people decide something like "well this question about this specific feat is already covered by this other question about combat in the third paragraph of one answer" which isn't good enough.
On timing: We prefer closing newer questions a dupe of older unless there's a major quality benefit the other way around. Some examples of when there's such a quality benefit:
- The older one isn't answered (or has bad answers), and the new one has good answers.
- The newer question is flat-out better written, easier to follow, and generally more useful for anyone interested in that line of inquiry. Answers to it are equal or better ("both have no answers" is equal).
- This typically occurs when we establish a high-quality canonical question to handle a subject straightforwardly that's previously been handled messily in various earlier questions.