What is a canonical tag, and when do I need one?

Duplicate URLs appear on almost every site without anyone deciding to create them. An online store in Rawalpindi might show the same kurta at /products/kurta, /collections/summer/products/kurta and /products/kurta?colour=blue. A campaign link might add tracking parameters. A page might load with and without a trailing slash, or on both http and https. To a person these are one page; to Google they are separate URLs competing for the same searches, splitting links and wasting crawl visits.

The canonical tag solves this by naming one preferred URL. It sits in the page's head as a link element with rel set to canonical, pointing to the address you want shown in results. Every duplicate points to that main URL, and the main URL points to itself. Shopify and most WordPress SEO plugins add canonicals automatically, which handles the common cases, but custom-built sites often forget them, and filter or search pages are frequently left uncovered.

Two cautions. First, Google treats the tag as a hint. If your canonical points to a page with different content, or your internal links and sitemap all favour another URL, Google may ignore your choice; Search Console reports this as 'Duplicate, Google chose different canonical than user'. Keep your signals consistent: link internally to the canonical version and list only canonical URLs in the sitemap. Second, a canonical is not a redirect. Visitors can still reach the duplicate, which is right for filters and tracking links. When an old page has been permanently replaced, a 301 redirect is the correct tool instead, because it moves both people and Google to the new address.