It's common for a site to have several pages listing the same set of products. For example, one page might display products sorted in alphabetical order, while other pages display the same products listed by price or by rating. For example:
http://www.example.com/product.php?i...678asfasdfasfd
http://www.example.com/product.php?i...678asfasdfasfd
If Google knows that these pages have the same content,
we may index only one version for our search results.
Our algorithms select the page we think best answers the user's query. Now, however, users can specify a canonical page to search engines by adding a <link> element with the attribute rel="canonical" to the <head> section of the non-canonical version of the page. Adding this link and attribute lets site owners identify sets of identical content and suggest to Google: "Of all these pages with identical content, this page is the most useful. Please prioritize it in search results."