The policy of the company I worked for was only to dupe to closed issues if those issues were Resolved -- if the duplicate issue was already closed Informational or N/A, we just closed the new one with the same status. This has advantages in avoiding researcher confusion, as illustrated here.
But that was a company policy, not an H1 policy. It's perfectly possible to dupe to a closed issue. (And of course, it's also possible that you get duped to an open issue which is later closed N/A, though that's pretty awkward. You kind of hope for N/A issues to be closed right away, not to stay open for long periods.)
And not duping to closed issues causes other issues -- it meant always having to leave an internal comment citing the other issue that this one was secretly a duplicate of.
Could you could state that the newly reported issue is both duplicate and that the original report was closed as N/A?
Not applicable typically means the reporter is free to try to argue that is in fact applicable, but by stating it's both duplicate and N/A neither the second reporter nor the company will spend further time arguing back and forth, as even if the issue was applicable the credit would go to the original reporter.
It looks like what happened here was that the issue was (explicitly) labeled a duplicate, and the original issue was (implicitly) N/A, which you can tell if you're familiar with the platform by the fact that the duplicate report cost reputation points.
This achieves the result you mention, that interest in litigating the report further is muted because it's a duplicate. Though you might want it recognized as applicable anyway because of the reputation effects, even if you're the duplicate.
I did once see a company receive a report that duplicated an earlier report that had been closed by mistake. When the new one prompted a reexamination, they reopened the earlier report and duped the new one to it. That struck me as pretty honorable compared to the easier path of leaving the closed report closed and just processing the new one as if it were new.
