When are questions which build upon each other considered duplicates?

Question

Each Stack Exchange site caters to a mix of experts and novices. This means that some (expert) questions presuppose knowledge which is being asked about in other (novice) questions. In other words, we sometimes have pairs of questions where one question is "building upon" knowledge contained in another question.

Sometimes when two questions build upon each other, one of them gets marked as a duplicate of the other. Marking a question as a duplicate prevents users from adding additional answers.

I'm asking this Meta question in order to seek guidance for users and moderators of the various SE sites, as to when questions which build upon each other should be regarded as duplicates, and to solicit advice about what other ways we can or should use to record the connection between such questions.

The original version of this post was illustrated with a particular question I asked on Linguistics Stack Exchange. I've updated it with some other examples I found by searching Stack Exchange for "This question has been marked as a duplicate". Here are the examples, including my original example. I've rephrased the questions to try to clarify their connections to each other.

1. English Language and Usage

   • Q1: Is "it" referring to something in this sentence?
   • Q2: What is "it" referring to?

2. CSS (Stack Overflow)

   • Q1: Why would someone want to make a table using CSS display:table properties rather than a regular HTML table?
   • Q2: How do I make a table using CSS display:table properties?

3. Python

   • Q1: How do I keep class data from being shared between instances? By creating an instance variable
   • Q2: How do I avoid sharing in an instance variable that refers to a mutable collection? By creating a copy of the collection

4. Linguistics

   • Q1: Why is Laam sometimes pronounced as the next letter in Arabic? Because the Arabic definite article Alif-Laam is assimilated if the next letter is a "sun" letter
   • Q2: Why do "sun" letters cause assimilation in Arabic? Because they are coronal consonants

5. Math

   • Q1: Why is Q[x,y]/<x^2+y^2-1> an integral domain? Because x^2+y^2-1 is irreducible in Q[x,y]
   • Q2: Why is x^2+y^2-1 irreducible in Q[x,y]?

6. Haskell (Stack Overflow)

   • Q1: Why is this value being evaluated twice in a declarative language? Because the compiler inferred a polymorphic type for the value and couldn't share it across occurrences
   • Q2: Why is the compiler inferring a non-polymorphic type for this value? Because of the monomorphism restriction

7. Space Exploration

   • Q1: What is the "specific impulse" of an engine? It's the effective exhaust velocity divided by the gravitational acceleration at the Earth's surface
   • Q2: Why do we use the gravitational acceleration at the Earth's surface when calculating the specific impulse of an engine?

In the Math and Python examples, Q2 was marked as a duplicate, but in the other examples Q1 was marked as a duplicate.

In the CSS example, the question was marked as a duplicate before it received an answer. However, in all the other examples an answer was given before the question was marked as a duplicate.

A commenter on the first version of this Meta question suggested that there could be no general rule and that I should go to the site-specific Meta for help. Perhaps that is still true for each of these examples. In any case I hope that the additional examples can give a bit more background as to what I am asking about here, than I had provided in my original question.