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
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.
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.