(Added in edit to explain someone else's comment and reference) ... it doesn't beg the question, it raises it. But putting that to one side ...

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.

Not sure if this has been suggested or not, but what about a list of "similar articles" below yours (based on a search of past articles based on title/content/etc) so that you can verify yours is unique?

It's an interesting idea, and could be useful. I think that finding a selection that matches your intended submission could be difficult. Would be cool, though.

Technically not difficult, but would add quite a bit of complexity to HN in order to fetch and store at least a fuzzy hash of the article bodies.

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.

You seem to be implying that this wikipedia page states that "begging the question" is improper English.

It's not.

From the page itself: "Some usage commentators deem such usage incorrect".

It also begs the question why don't comments regarding duplicates on duplicate submissions automatically get de-duped?

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.