Canonical

A canonical link tag in the header of a webpage's HTML tells search engines which URL is the official authoritative source for the content. If the same page can be accessed with two or more URLs, then this will make sure that your preferred URL is listed in search engine results and that search engines do not penalize you for having duplicate content.

<link rel="canonical" href="https://www.example.com/article.html">

Maybe you have two very similar pages on your website. By listing one of them as the canonical source, you can consolidate your search engine presence into one of those pages instead of having it split between the two. This can improve your search engine ranking.

Even if you don't have similar pages, you may have multiple ways to access the same content. Perhaps "www.example.com" and "example.com" lead to the same page. Or maybe the same page can be access by both http:// and https://. These appear as duplicates to search engines and a canonical link will help (although you should also setup server redirects in these cases). Your page may use a query string at the end of the URL like this: www.example.com/article.html?page=2&session=abc123. Some of these parameters may lead to different content on the page, but other parameters may not and this can result in different URLs pointing to the same content. Use a canonical link to disambiguate it for search engines.

This test will look for a canonical link tag that matches the current page.