No; a question should not be closed as a duplicate of a different one that assumes the answer to the first question as a premise.
There are plenty of Q&As related to one another on RPG.SE, where:
- Question #1 asks whether X is true (for instance), or what the
answer to Y is.
- Question #2 assumes/concludes (correctly or incorrectly) that X is
true/false, or that the answer to Y is Z; usually, it treats that
claim as fact (i.e. as a premise of the question).
The numbering here doesn't necessarily imply anything about the order in which the questions are asked; in your case, Question #2 already existed before Question #1 was asked.
In fact, I suspect this issue would be even clearer if Question #1 were asked first; it'd make no sense to close a question that asks "Is X true?" as a duplicate of a later question that asks, "Since X is true, can I do X2?". In fact, that premise in Question #2 is often explicitly based on the answer(s) to Question #1, and links back to the first Q&A as the source of that information.
For instance, the Q&A What is the best damage you can do with a 9th level Magic Missile? states that the magic missile involves a single roll that determines the damage of every dart of force from the spell; you wouldn't close the Q&A How many times do you roll damage for Magic Missile? as a duplicate of it. That's true regardless of the order in which the two questions were asked, but especially because the "best damage" question directly links to the "how many times do you roll damage" question as the source of its assertion that you only roll once for the spell's damage.
In essence, Question #1 can never be a duplicate of another question that already assumes that what the answer to Question #1 is. Question #2's claim as to the answer to Question #1 is may or may not be correct; if the premise of Question #2 is faulty, its answers should point that out - but regardless, the questions are clearly not the same.
In your case, your Q&A Is there any way to give proper disadvantage on saving throws against spell effects to a creature with Magic Resistance? is Question #2 (per my explanation above), though it was asked first, and Somebody The Stranger's new Q&A Can magic resistance be overcome? is Question #1.
Somebody The Stranger's question asks "would effects that impose disadvantage on saving throws [...] nullify [the Magic Resistance monster trait by canceling its advantage]?"
Meanwhile, your question already assumes that the answer to that is "yes", and instead asks if there's a way to actually give the target disadvantage:
Suppose I wanted to weaken the Archmage's resistance to magic - I use
some effect that gives disadvantage on the saving throw for my spell.
Normally, this would balance out to a straight roll, as the rules for
advantage and disadvantage say:
[...]
This seems to set a baseline for a straight roll on saves against
magical effects, that the Archmage can never roll at disadvantage
against magical effects.
But is there a way to get around this?
It's clear that what you're asking is not what Somebody The Stranger is asking. You already understand the interaction of Magic Resistance with disadvantage, and are looking for a way to actually get around that to give them disadvantage (that's not canceled out by advantage) - but they're asking whether they're correctly understanding the interaction of Magic Resistance with disadvantage in the first place.
Their question is definitely not a duplicate of yours - though you could use the citation from that portion of your question as the basis of an answer to their question. (That said, the titles of the two Q&As could be edited to more clearly indicate what they're asking, so that it's clear that they're asking two different things.)
Note: Somebody The Stranger has since clarified that they were actually unaware of the "advantage and disadvantage cancel out" rule, so it's definitely different from your question. I've edited both questions' titles to more clearly distinguish what each is asking:
