Why does this matter?
When multiple pages have the same or significantly similar content, search
engines consider them duplicate versions of the same page. For example, desktop
and mobile versions of a product page are often considered duplicates.
Search engines select one of the pages as the primary, canonical, version and crawl that one more, while crawling the other ones less frequently. Crawling is how search engines update their index of content on the web, and by providing search engines information about your preferred canonical URL you're helping search engines display the correct URL to users.
Lighthouse displays the following failed audit if your duplicate URLs are difficult for search engines to understand: "Document doesn't have a valid rel=canonical".
Decide which URL is the canonical version
First, decide which URL should be the canonical version of your content. Make
sure that the canonical URL is not blocked from crawling with a
file, not blocked from indexing with a robots meta element, and publicly
accessible. Ideally, use HTTPS URLs instead of HTTP URLs if you have a choice.
If you use hreflang
links, make sure that the
canonical URL points to the proper page for that respective language or country.
Also, watch out for the following problems:
- Don't point the canonical URL to a different domain. While Google allows this, Yahoo and Bing don't allow it.
- Don't point lower-level pages to the site's root page, unless the content is the same.
Specify the canonical link
There are two ways you can specify a canonical link:
link rel=canonicalelement in the
<head>of a page
- Link header in the HTTP response
For a list of pros and cons, see Google's guide to duplicate URLs.
Option 1. Add a canonical link element to the head of the HTML
<link rel="canonical" href="https://copycat.com/"/>
Option 2. Add Link header to the HTTP response
Link: https://copycat.com/; rel=canonical
Here's a full example of what the
<head> should include.
<title>Mary's Maple Bar Fast-Baking recipe</title>
<meta name="Description" content="Mary's maple bar
recipe is simple and sweet, with just a touch of
serendipity. Topped with bacon, this sticky donut
is to die for.">
Run the Lighthouse SEO Audit (Lighthouse > Options > SEO) and look for the results of the audit Document doesn't have a valid rel=canonical.
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