A canonical URL identifies the preferred address among URLs with identical or very similar content. On a Joomla website, it helps align addresses created by parameters, different menu items or alternative routes.
How canonical URLs are declared
For HTML, a link element with rel="canonical" in the head section identifies the preferred URL. A Link HTTP header can serve other resource formats. The target should be accessible and represent the page content.
A canonical declaration does not redirect visitors. The browser keeps displaying the original URL, while a 301 or 308 redirect sends it to another address. Search engines treat canonical declarations as signals and may select a different URL when other signals conflict.
Joomla considerations
Before configuring canonical URLs in Joomla, compare the actual content of the proposed duplicates. A category page, an individual article and the next page of a listing are not duplicates simply because they share a topic. A translation should normally have its own canonical address.
After configuring Joomla, inspect the source HTML, response headers, internal links and sitemap. Multiple extensions can produce conflicting canonical declarations, and a target returning 404 is unsuitable. Internal links and sitemap entries should consistently point to the preferred version.
Conclusion
Canonical URLs help consolidate signals for similar pages, but require checks of content and address consistency. Use a redirect when a page has permanently moved.