So why does that mean that duplicates are dangerous? Because of updates. When someone answered a question about how to do something in Python (but it was 2008 so it was written in Python2) SO had ways to get a more correct, up-to-date answer to that question written in 2015 (and then again in 2019) and get that upvoted, and moderators could reward that new answer by editing the original etc.
That is why duplicates were a major threat: if the same question is asked and answered thousands of times, no one is going to go do the work to update all of those answers all across the site. Those lurkers are now dependent on the whims of Google as to which of the many answers you get taken to, and whether it has the latest answer or some answer that stopped working years ago.
And that is why they were so hostile to duplicate questions.
They were so fixated on solving that that they failed to realize: training all their power users to grief anyone who didn't behave like a power user off the site was detrimental (to having a site with non-power users). Everyone came for a question and answer site, but when it transformed it into a "question and get downvoted and modded into oblivion" site, everyone left.
AI put the final nail in the coffin, but SO was dead before AI arrived ... from this self-inflicted wound.
To extend this, the eventual problem is that you eventually lose all of your new users, which means you lose the extremely valuable intermediate users (ones that know enough to ask complex questions, but not so much that they can figure out the answer).