A successful Git branching model
nvie.com
nvie.com
They're not all the same URL, and when a submission is old enough it's no longer tested against. Minor variations in the URL will also thwart the dup detector.
Several suggestions have been made in the past about improving the dup detector, but it's unlikely they'll be implemented.
From there it's mostly a case for a bayesian classifier (LSI) and some elbow grease.
FWIW, the "similar articles" footer on most blogs is implemented that way.
It's not.
From the page itself: "Some usage commentators deem such usage incorrect".
The fun aside, it boils down a matter of ROI on effort. It is not trivial to detect duplication (save exact matches) on URLs. If the community is enabled to deal with it in a non-automated way, it is not worth the trouble dealing with all the threads about false positives and other similar troubles.
Some links like this and the comments that ensue are 'evergreen' material, but by continually being resubmitted at a later date the chance of conversation happening decreases and its time on the homepage shrinks while it creates a sort of broken window effect for those of us who haven't seen it for the first (or 6th) time.
and the code: