Canonical tag
A canonical tag is a link element that names the preferred URL for a page, telling search engines which version to index when the same content is reachable at more than one address.
Duplicate addresses are normal and mostly accidental: tracking parameters, trailing slashes, http against https, with and without www, print views, faceted navigation. Without a canonical, ranking signals split across the variants.
The common failure is a canonical that points somewhere else by mistake, usually because a template hard-codes one URL. That is more damaging than a missing canonical, because it actively tells the engine to drop the page.
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