Glossary
Canonicalisation is declaring which URL is the authoritative version of a page when several URLs serve the same or similar content, so search engines consolidate signals onto one address instead of splitting them.
Tracking parameters, filter combinations, uppercase and lowercase paths, with and without a trailing slash, print views can each serve the same content at a different address. Left undeclared, links and relevance signals divide between them and none of the versions is as strong as the single page should have been.
Search engines treat it as one signal among several, and will ignore it if other evidence disagrees: internal links pointing elsewhere, a sitemap listing a different URL, redirects contradicting the tag.
The canonical, the internal links, the sitemap entry and any redirects should all name the same URL. When they agree, the declaration is almost always honoured; when they conflict, the engine picks for you.
There is no penalty for ordinary duplication. The cost is dilution: signals split across several URLs so no single version is as strong as it should be. Canonicalisation consolidates them.
Yes. A self-referencing canonical on every page is good practice. It removes ambiguity when a URL is reached with parameters appended and costs nothing.
Canonicalisation is declaring which URL is the authoritative version of a page when several URLs serve the same or similar content, so search engines consolidate signals onto one address instead of splitting them.
