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.