Real duplicate-content problems are rarely deliberate — they come from e-commerce filters generating slightly different URLs for the same product, tracking parameters appended to otherwise-identical pages, or a CMS serving the same content at both a trailing-slash and non-trailing-slash URL. A canonical tag tells search engines which of those variants is the "real" one to index.
One canonical tag, pointing one direction
A page should have exactly one canonical tag, and it should point to the preferred version of that content — either itself (a self-referencing canonical, which is normal and expected) or another URL if this page is a variant. Having more than one canonical tag on a page leaves search engines to either ignore all of them or pick one unpredictably.
It's easy for platform bugs to add a second one silently
A common real-world cause of duplicate canonical tags is a CMS theme or plugin adding its own canonical tag independently of the one your SEO plugin or template already generates — both technically valid tags, both present, and now there are two.
TeckForge's Canonical Tag Checker scans a page's raw HTML for canonical tags, flags duplicates immediately, and shows exactly which URL each one points to.
