A major problem for search engines is to determine the original source for documents that are available on multiple URLs. Content duplication can happen in many ways, including:
Duplication due to GET-parameters
Duplication with multiple URLs due to CMS
Duplication due to accessibility on different hosts/protocols
Duplication due to print versions of websites
Duplicate content issues occur when the same content is accessible from multiple URLs. For example, http://www.example.com/page.html would be considered by search engines to be an entirely different page from http://www.example.com/page.html?parameter=1, even though both URLs may reference the same content. In February 2009, Google, Yahoo and Microsoft announced support for the canonical link element, which can be inserted into the section of a web page, to allow webmasters to prevent these issues. The canonical link element helps webmasters make clear to the search engines which page should be credited as the original.
How search engines handle rel=canonical
Search engines try to utilize canonical link definitions as an output filter for their search results. If multiple URLs contain the same content in the result set, the canonical link URL definitions will likely be incorporated to determine the original source of the content. ″For example, when Google finds identical content instances, it decides to show one of them. Its choice of the resource to display in the search results will depend upon the search query.″ According to Google, the canonical link element is not considered to be a directive, but rather a hint that the ranking algorithm will "honor strongly." While the canonical link element has its benefits, Matt Cutts, then the head of Google's webspamteam, has said that the search engine prefers the use of 301 redirects. Cutts said the preference for redirects is because Google's spiders can choose to ignore a canonical link element if they deem it more beneficial to do so.
Implementation
The canonical link element can be either used in the HTML or sent with the HTTP header of a document. For non HTML documents, the HTTP header is an alternate way to set a canonical URL. By the HTML 5 standard, the HTML element must be within the section of the document.
Examples
HTML
Below you can see an example of HTML code that utilises the rel=canonical inside the tags. The code could be used on a page such as https://example.com/page.php?parameter=1to tell search engines that the https://example.com/page.php is the preferred version of the webpage.
