What is a canonical URL?
In short
A canonical URL is the preferred address of a page when the same content is reachable at several addresses. You declare it with a link rel="canonical" tag so search engines combine signals on one URL.
<link rel="canonical" href="https://example.com/guides/robots-txt">
Every indexable page should have a canonical link, usually pointing to itself. It's a strong hint, not a command: search engines may choose a different canonical if other signals disagree.
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