An improvement to the Hacker news website would be to compute and compare hashes of weblinks so that the same link is not reposted multiple times. Later posters of links could then be redirected to the first post of a link.

It wouldn't work - it's trivial to add meaningless query parameters or anchors that would change the hash but still lead to the same content. And stripping that wouldn't work because some sites use them to route to content.

What might work is hashing the text and outbound link content submitted pages of, and building something like a similarity index of text, metadata and a graph of links, but that would probably still be fragile, and definitely be too much effort for a site with as little traffic as this.

They could capture the canonical URL from the meta tags in the page. I don't think they do currently.

Assuming a site has one, although most news sites probably do. Facebook Open Graph and other social media tags are worth looking for as well. Unfortunately, they're not always trustworthy.

Would still pick up some of the low hanging fruit though which is better than nothing.

I think they do. I have submitted a link before and been redirected to the existing comments page.

It does that, but only if the other submission has been very recently and/or(?) has points over a threshold - otherwise duplicate submissions are explicitly allowed.

This behavior happens already, but only if the link has gotten sufficient attention.

Why even hash the weblinks? You can just store them directly no?