Is there a simple term for the question this one was closed as a dupe of other than "the question this one was closed as a dupe of", because "the question this one was closed as a dupe of" is pretty long and unwieldy and much in need of a simpler term than "the question this one was closed as a dupe of".
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It's not a single word, but the duplicated question seems like an obvious answer.
In the context of SE questions there's often a certain amount of debate over whether the later question is identical to the one cited as a duplicate, so I tend to avoid the term original.
Admittedly this approach deprives me of a convenient noun form, but I think it's less contentious to refer to it as the earlier question. But if you really want a noun I suggest antecedent.
Ideally on SE one would like to refer to the earlier question as the archetype, to imply we've already got a generic/canonical answer for any questions similar to the one currently being flagged as a later duplicate. Sadly, that isn't always the case, so people can often argue theirs isn't a duplicate.
In programming, I've sometimes heard original used as a relative term, as in "the original of the new bug is a year old." Perhaps this extends to online Q&A discussions.
Since each question is closed as a duplicate of exactly one, the relationship is unique and you can probably refer to it in context as simply the duplicate, even if that's somewhat backwards.
The notion of duplication and redundancy are very often justified in prose, but I agree that a pithy term is ideal here. Specifically, we want a noun describing "that which is immediately deprecated because it contains a strict subset of another preceding [concept or idea]."
Picking through Everything2's lexicon on writeup deletion (a site based on the concepts of social karma and article deprecation far predating StackOverflow and StackExchange), their chosen phrasing for this case is, loosely speaking, when one's article is "superseded by an existing writeup."
This phrasing lacks precision, however. The transitive verb supersede strictly means (according to Merriam-Webster):
Its etymology (as addressed in this English.SE answer) is, tersely:
I say this "lacks precision" because the order of operations is reversed in the case of a question or writeup that is instantly deprecated by another, previous question or writeup. It isn't a case of the previous "forcing out" or "displacing" the new one as inferior (implying subsequent action), so much as the new duplicate is, in fact, deprecating itself by virtue of not deferring to the previous one.
A word suggestion I would put forth here is not supersede, then, but subsede. This isn't currently a word in the dictionary. But, by our prior understanding of Latin prefixes and verbs, this would etymologically mean:
Or, literally, "sit under or undermine" the subsedee, as the subsedent or subsedent question.
But, I digress. The authoritative, original, or (bending the rules again) supersedent question should be expressively sufficient for your purposes when referring to the original authoritative source deprecating a now-unoriginal question or idea. I'd shy away from using the duplicated question only insofar as it's technically unclear, without additional context, which of the now-duplicate questions you're referring to.
Given that you seem to spend most of your time on programmers forums, they'll probably recognise the word deduplicate.
— The Standard Repository of All Knowledge (a.k.a. Wikipedia).
So you could say:
But do be careful not to cause offense! ;-)