luceos the 2 common duplicate page issues I know of are:
Multiple domains: usually fixed by the web host automatically, by an htaccess or nging rewrite, or by my Canonical Url Redirect extension.
Multiple paths to same content: Flarum extensions have the tools necessary to handle this by setting the canonical value on the Document object when creating custom pages. But I don't even know of any extension that intentionally creates duplicate URLs and really needs this value. It's mostly useful for the Flarum pagination to reduce the number of indexed discussion URLs.
Nowadays with AI and stuff it wouldn't surprise me if Google or others want to penalize having many pages that look too much the same, but at the same time, sitemaps, glossaries and tag indexes have been extremely common for as long as the web has existed.
Perhaps this issue is entirely about the pages being different but not having a distinct enough title or meta description, and search engines not knowing how to present them to the user with enough context? For this particular issue, the only solution would be to change the extension. The prettiest implementation would probably to copy what the Tags extension does if that's not already the case. If those URLs are meant to be perma-linked and indexed, they should have their own URL prefix, and then the page content can re-use the discussion index template like for Tags. The Tag detail page is essentially just a search result page, but it hides away the search and changes the content around the results (page title, hero, etc.).
