On HN, reposts are fine (which is to say, aren't treated as dupes) when a story hasn't had significant attention in the last year or so. This is in the FAQ: https://news.ycombinator.com/newsfaq.html.
But it's great to link to examples of past threads, because readers like to look at them—so this was a good contribution! just not with the tag "duplicate".
Edit: basically, there are two disjoint cases when it makes sense to link to a previous submission of the same story:
(1) when a prior submission of the same story had significant attention in the last year or so, then linking to it with the word "duplicate" or "dupe" is helpful;
or
(2) when a prior submission had significant attention more than a year ago, then linking to it with the phrase "past discussion" or "discussed at" (or something like that) is helpful.
(And for completeness: when a submission has not gotten significant discussion, it's best not to link to it at all. There are plenty of those in the archives, and they're mostly harmless.)
Thanks for that last note, it's been my argument for a while and I've had a few discussions with people on that point, e.g., <https://news.ycombinator.com/item?id=40849927>. I'll try to remember this comment to share for those cases.
But it's great to link to examples of past threads, because readers like to look at them—so this was a good contribution! just not with the tag "duplicate".
Edit: basically, there are two disjoint cases when it makes sense to link to a previous submission of the same story:
(1) when a prior submission of the same story had significant attention in the last year or so, then linking to it with the word "duplicate" or "dupe" is helpful;
or
(2) when a prior submission had significant attention more than a year ago, then linking to it with the phrase "past discussion" or "discussed at" (or something like that) is helpful.
(And for completeness: when a submission has not gotten significant discussion, it's best not to link to it at all. There are plenty of those in the archives, and they're mostly harmless.)