What to do when a new question is a superset of an older answered question?

Question

This old question asks why Pesach and Sukkot last seven days.

This recent question asks why Pesach and Sukkot last seven days and (in contrast) Shavuot lasts only one.

What is site policy in this case?

Closing the old question as a duplicate seems pretty unfair and non-intuitive.

Closing the new question as a duplicate also seems to be wrong, as the old question is not asking for answers about Shavuot.

I believe there is some way for moderators to merge questions, but I'm not sure if that would be appropriate here either, as answers to the first question don't fully answer the second, and answers to the second are providing more information than was requested by the first.

Maybe the answer is simply to leave both questions as they are, with a note in the comments?

Answer 1

I'm not sure if this generalizes to all such cases, but in many such cases, especially before any answers have been posted to the new question, the best thing to do is to edit the new question to link to the old question and focus on what the new question adds to the discussion.

So, for example, I'd alter the new question you linked to something like

Why is Shavout one day, unlike other holidays, which are seven?

The Three regalim got me thinking.

This previous post discusses why Sukkot and Pesach are seven days long. Why is Shavout, by contrast, only one day long?

I'm hesitant to do that now, though, as the new question already has an upvoted answer that addresses both the old and new parts of the question, which could make it unfair to change the scope of the question out from under it. I will ask the author to consider reposting the relevant parts of that answer as an answer to the older question and then narrowing its scope locally. Regarding this, too, I'm not sure if this approach generalizes to all such cases.

Answer 2

For a true superset, close as a duplicate until any duplicate parts are removed (or edited to reference/summarize as in Isaac's answer).

Duplication is to reduce redundant effort. This fits the bill exactly. It's also not permanent for cases like this.

Consider if "What's the blessing on apples and oranges?" was asked after "What's the blessing on oranges?" to see an intuitive application of the above rule.