I believe the dupe detector is left purposely "leaky."
I don't understand what's so wrong with dupes, especially the amount of effort you've put into this one. I don't see every headline on HN, neither do you, nor does anyone (I hope). What is your methodology for trawling through the archives to find interesting things you might have missed? Some people like having a chance for them to pop up again, which usually indicates that they're high quality. And inevitably someone will show up in the comments and link all the old discussions and then people can review those and also start a new discussion! And if you've already seen it, you can skip it! What's the issue?
Obviously, a massive flood of dupes is not desirable, so there is balance to be found, but I think they hit it pretty well. They also have to factor in the wide range of reading habits.