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There is no one, over-arching rule across the network. 
Communities are self-governing (within the limits of what we can host).  I agree with [this answer](https://meta.codidact.com/posts/292587/292606#answer-292606).

Personally, I think duplicates should be evaluated question to question, not considering answers.  If two completely different questions were to nonetheless have the same solution, that would not make them duplicates.  It might make them *related* (and I've long wanted a better way than comments to indicate related questions), but if a person asking, or seeing, question A cannot read question B and clearly see that yes those are the same question, it's not a duplicate in my opinion.  The expectations around cross-community duplicates, like scope boundaries, are worked out by mutual consent of the communities involved.

But that's my opinion.  Any community on our network could have stronger or weaker rules.  Heck, communities can rewrite, supplement, or even remove the "duplicate" close reason.  Here on Meta we added "superseded by" to convey "same issue but go over there instead", because "duplicate" wasn't quite right for those cases.  (We also have the off-the-shelf duplicate reason.)

Some communities are here to build a carefully-organized repository of knowledge with clear boundaries and no overlap across questions or answers.  They are free to do that.  Others are here to provide answers with the context that's needed for the particular question -- answers on two different questions might both say (or link to) "do X" but include explanations about *why* to do X that are completely different because the questions are different.  They are free to do that too.  Some communities are here more for the satisfaction of the participants than a goal of building a timeless repository (for example, puzzle/challenge-focused communities).  And they, too, are free to do that.

One size doesn't fit all.  I want to help each of our communities best serve its goals and its participants.