Even if that is correct it does not make it a just way of fighting for your cause. Voting for articles on the front-page of HN is equivalent to a vote in a democracy. If you removed the opposing party candidate from the ballot you might reach your short-term goal, but at the cost of a functioning democracy.

In this case the article was marked as a dupe, but this specific article was not posted twice and previous articles on the subject was quickly voted off the frontpage by people like you despite high interest. This didn't happen to articles critical of Damore, so this seems to be exclusively a social justice tactic.

Edit: explained better why talking about voting is relevant as an analogy

I expect the moderators, or whoever it is that looks after this stuff, decided that since it was discussing the same case as the other articles then it counted as one in principle.

Sometimes multiple articles relating to a particular issue are posted. When these links don't attract much discussion, this isn't much of a problem. But when they do, it's probably best to try to centralize the discussion, lest the entire front page get filled up multiple copies of the same stuff.

This has happened before in the past and it's a bit dull if it's something in which you have zero interest.

Those articles were all flagged off the front page, not many people got the chance to see them in the first place. Whoever claimed this was a dupe was just being dishonest and possibly malicious.

I understand your point that if a submission hasn't been widely seen subsequent submissions shouldn't be marked as a dupe. That said, HN is curated, both by the mods and members. It's not purely a popularity contest or a democracy. Members may (and do) disagree on the curation methods, but the curation methods don't require a certain threshold of visibility for any given piece before taking effect.

Marking a submission as a dupe serves a different purpose: it provides a pointer to the "canonical" submission for a given discussion, and not just for those that have spent some threshold of time on the front page.

You may very well disagree with the effects of HN curation in general or for this submission in particular; however, I think it's valuable to recognize that marking submissions as dupes and flagging/downweighting are independent.

Marking of a submission as a dupe that was previously itself flagged off the front page is patently dishonest. Of course, HN is curated, its members can curate however they want, for whatever reasons they want, and those reasons are not always going to be good ones, which in this case they obviously aren't.

You can’t both flag an article on a topic off the frontpage so few in the audience see it and get to claim that another article on the topic is a dupe for the audience.