Canonicalisation is the process of choosing the main URL for a piece of content. A canonical URL is the main URL that search engines select from a group of duplicate pages. This helps search engines display only one version of the content in search results.
There are several reasons why a site might have duplicate content:
- Region variants: Same content for different regions (e.g., USA and UK) with different URLs.
- Device variants: Same content for mobile and desktop versions.
- Protocol variants: Same content on HTTP and HTTPS versions of a site.
- Site functions: Results from sorting and filtering on a category page.
- Accidental variants: Demo versions of a site left accessible to search engines.
It's normal to have some duplicate content on a site. However, if the same content is available through many different URLs, it can confuse users (they might not know which page to choose) and make it harder to track how your content performs in search results.
Helpful resources
If you want more information on the canonical tag read Google's - 'What is canonicalization' guide.