Zulip links to specific discussions put lots of information in the anchors, and the only thing before the anchor is the top-level domain (such as rust-lang.zulipchat.com). Discourse seems to ignore the anchor when doing duplicate link detection, which results in messages like this:
I took a look at this and it’s quite strange but I can’t reproduce it. There is no obvious code that strips off the URL hash when normalizing links. We do strip off trailing slashes, and I found a bug where we should be ignoring case in the URL but aren’t.
In the example you linked to, it looks like the link was oneboxed to a /login link. In that case all login links would be the same. Is that the problem?