When the same content (a substantive portion of text or other page components) appears on two URLs, Google sees it as duplicate content. The content is considered duplicate whether it appears on the same domain or separate domains.
There are common non-malicious practices that result in duplicate content — for example, discussion forums that generate a different, more pared-down version of the forum for mobile devices may end up generating two separate URLs.
Duplicate content can hurt search rankings because it causes a number of problems for search engines:
- The search engine isn’t sure which URL to rank in results for a specific query because it can’t tell which page is better.
- The search engine doesn’t know how to split the value of the content (as judged by keywords, anchor text, link equity, trust, and more) between the URLs.
Typically, search engines deal with duplicate content by selecting one of the two pages. In cases where you’re aware that you have duplicate content, you can use canonicalized tags to tell Google which one to rank.