I've discovered an issue in Joomla 5.3.2 and 5.4 where menu items set to "All languages" are incorrectly accessible and rendered with language-specific URL prefixes, creating duplicate content and SEO issues.
Environment:

Joomla 5.4
Multilingual site setup (IT default, EN secondary)
SEF URLs enabled
Language Filter plugin enabled
No other plugins that could interfere

Steps to reproduce:

Set up a multilingual Joomla site
Create a menu item with alias "home" and set language to "All"
Make it the default Home item (required by Joomla)
Create language-specific menus with their own home items
Access the site with URL: /home

Expected behavior:

/home should redirect to language homepage /it/ or /en/ (based on browser/cookie)
URLs like /it/home or /en/home should either:

Return 404 error (preferred), OR
Redirect to /it/ or /en/ respectively

"All languages" menu items should NOT be accessible with language prefixes

Actual behavior:

/home redirects to /it/home or /en/home (based on language detection)
/it/home and /en/home both display the "All languages" menu content
Both URLs show the same "Home" content instead of 404
This creates duplicate content (same "All languages" content accessible via multiple language-prefixed URLs)

Impact:

SEO penalty for duplicate content
Search engines index multiple URLs with identical content
Incorrect routing behavior breaks multilingual URL structure
Confuses visitors who expect language-specific content

Additional notes:

No menu module is displaying this "All languages" menu item
Changing the menu item alias doesn’t fix the issue; the new alias inherits the problem.
The "All languages" content should never be accessible with language prefixes
This appears to be a core routing/Language Filter bug

I'm aware that this problem doesn’t seem to occur in Joomla 6 (I have only one site running that version, but many on 5.x).