When Google finds multiple URLs on the same site that show essentially the same page contents, it considers these URLs to be duplicates. For example, if one URL on a site shows a list of dresses grouped by size, while another URL on the site shows the same dresses grouped by color, and a third URL is the same list, optimized for display on mobile devices.
Google analyzes a group of duplicate URLs and chooses a canonical URL. The canonical URL is the most representative version of that page and also displays well on the "standard device type" used by most site visitors (for most sites, this is a mobile device). Duplicate URLs aren't usually shown in Search results unless they are alternate URLs. Alternate URLs are a subset of duplicate URLs that are customized for specific devices or languages, and are served to users when appropriate for the user's device or language.