If you have a single page accessible by multiple URLs, or different pages with similar content (for example, a page with both a mobile and a desktop version), you should explicitly tell Google which URL is authoritative (canonical) for that page. If you do not do this explicitly, Google will make the choice for you, or might consider them both of equal weight, which might lead to unwanted behavior, as explained in the next section.
Note that although we recommend that you specify a canonical page for duplicate URLs or versions, Google might algorithmically choose a different page as canonical for various reasons such as performance or content.