Say your website can be reached at hendriksdigital.com but also at www.hendriksdigital.com. For visitors, that's the same thing. For Google, those are technically two different URLs with the same content.
Without clear instructions, Google may wonder: "which of the two should I show in the search results?" That's what we call 'duplicate content'. Google then splits the ranking power across both URLs, and your one page performs worse than it would if everything sat on a single version.
What does a canonical URL do?
The canonical URL is a small piece of code in the header of your page that says: "dear Google, the official version of this page is X." Google respects that and only shows that version in the search results. All the ranking power gathers on one URL.
Where do you often see duplicate content?
- www vs non-www: hendriksdigital.com vs www.hendriksdigital.com
- HTTP vs HTTPS: http://site.nl vs https://site.nl
- Trailing slash: /diensten vs /diensten/
- URL parameters: /product?kleur=rood vs /product?kleur=rood&sort=prijs
- Online stores: a product listed in several categories and therefore reachable at different URLs
When do you need to do something about it?
For most modern WordPress websites, this is set up correctly by default. If you use an SEO plugin (Yoast, Rank Math), it automatically adds a canonical to every page.
Do watch out with:
- Online stores with lots of product variants (color, size, filters)
- Sites with language or regional versions
- Blogs where the same article appears in several categories
- Hand-built sites without an SEO plugin
How do you check if it's right?
Right-click anywhere on your page, click 'view page source' and search for 'canonical'. You should see one line with <link rel="canonical" href="..." />. That URL is what Google treats as the 'official version'.
If the canonical points to the correct URL of your page: great, you're done. If it points to a different page than the one you're on: that's a problem, so get in touch with your web developer.