14 November 2026

Quick answer A canonical tag tells search engines which URL is the preferred version when the same or very similar content appears at several addresses. Use self-referencing canonicals on normal pages, point duplicates and variants to the main version, keep your internal links and sitemap consistent with it, and never canonicalise to a redirected or noindex page. Last updated: November 2026 Duplicate content is one of the most misunderstood problems in SEO. Google does not issue a penalty for ordinary duplicates, but it does have to choose one version to index, and it may not choose the one you want. Meanwhile, links and signals get split between versions, weakening each. Canonical tags are the standard tool for guiding that choice. They are simple in theory and surprisingly easy to get wrong, particularly on WordPress, WooCommerce and Shopify sites where plugins and themes generate many URL variations. This guide explains how canonicals work, where duplicates come from, the right way to use them, the mistakes that cause lost rankings, and how to audit your own site. In this guide What a canonical tag is Where duplicate content comes from Self-referencing canonicals Canonicals for duplicates and variants Canonical, 301 redirect or noindex: which to use? Common canonical mistakes WordPress, WooCommerce and Shopify specifics How to audit canonicals on your site A worked example Good practice checklist When duplicates do not matter FAQs What a canonical tag is A canonical tag is a line in the head of a page, written as a link element with rel=”canonical” and an href pointing to the preferred URL. It says: if you find this content at several addresses, please treat this one as the main version. It is a hint, not a directive. Google may ignore it if other signals disagree, for example if internal links, redirects and sitemaps all point to a different URL. That is why consistency matters more than the tag itself. Where duplicate content comes from HTTP and HTTPS, www and non-www, and trailing slash variations. URL parameters for tracking, sorting and filtering, such as ?utm_source or ?sort=price. Product variants and categories that show the same product at multiple paths. Printer-friendly pages, AMP versions and paginated series. Syndicated content republished on other websites. CMS settings that create archives, tags and attachment pages with thin or repeated text. Self-referencing canonicals Most SEO plugins add a canonical tag to every page that points to itself. This is good practice, because it protects against accidental parameter variants being treated as separate pages. A page at /services/ with a canonical to /services/ is saying “this is the original”. Check that your self-referencing canonicals use the exact preferred form: HTTPS, your chosen www or non-www version, and your chosen trailing slash style. A mismatch creates confusion that your own tag cannot fix. Canonicals for duplicates and variants When pages are near-duplicates, such as the same product in different colours with identical descriptions, you can canonical the variants to the main product page. When content is syndicated to another site, ask them to include a canonical pointing back to your original. Do not canonicalise pages that are genuinely different, even if they are similar. If a page for “boiler repair Leeds” canonicals to “boiler repair Bradford”, you tell Google to ignore the Leeds page. Each page that should rank in its own right needs its own self-referencing canonical. Canonical, 301 redirect or noindex: which to use? Choose the right tool for the situation. Use a 301 redirect when an old URL should no longer exist, such as after a page move or merging content. Use a canonical when the duplicate must stay accessible to visitors, for example a tracking URL or a filtered view. Use noindex when a page should be available to users but not appear in search at all, such as thank-you pages. Do not combine conflicting signals on the same page. A page with a canonical pointing elsewhere and a noindex tag, or one that redirects and also carries a canonical to itself, sends mixed messages and can cause unpredictable indexing. Common canonical mistakes Canonicalising to a URL that redirects, returns 404 or is blocked in robots.txt. Pointing every page to the homepage, which tells Google to ignore them all. Using relative or malformed URLs, including missing protocols. Having two different canonical tags on one page, for example from a theme and a plugin. Canonicalising paginated pages to page one, which can hide deeper content. Leaving staging URLs in canonical tags after launch. WordPress, WooCommerce and Shopify specifics On WordPress, Yoast and Rank Math handle canonicals automatically, but you can override them per page. Watch for plugin conflicts that output duplicates, and for attachment pages that create thin URLs. On WooCommerce, make sure variations and filtered URLs canonical to the main product or category. On Shopify, products can be reached through both /products/name and /collections/x/products/name. Shopify themes normally canonical to the /products/ version, which is correct, but your internal links should use that version too. Our Shopify build service sets this up properly from the start. How to audit canonicals on your site Crawl your site with a tool such as Screaming Frog and review the canonical column. Look for pages with missing canonicals, canonicals to non-200 URLs, canonicals pointing to a different page and multiple canonicals. In Search Console, the Pages report flags “Alternate page with proper canonical tag” and “Duplicate without user-selected canonical”, which show how Google interpreted your signals. The URL Inspection tool shows both the canonical you declared and the one Google chose. If they differ, investigate internal links, redirects and sitemap entries that may be pushing Google to another version. A worked example A Bristol furniture shop finds the same sofa page at /sofas/oslo, /sofas/oslo?colour=grey and /sale/oslo, each with different internal links. Google indexes the sale URL, which has less authority than the main product page, and rankings stay low. The shop sets the main product page as canonical on all three, updates internal links and the sitemap to