A Google Webmaster Help thread has discussion around a webmaster who noticed Google is treating his mobile site as a duplicate site to his main site. This user has his mobile site on a subdomain of m.domain.com, where his main site is on www.domain.com. The issue is that the content is exactly the same on m.domain.com as compared to www.domain.com.
So what can you do to solve the duplicate content issue?
In my opinion, the best way to provide a mobile version of your site is to use CSS. As Phil Payne said in the thread, "code each page once (in valid XHTML Strict) and use CSS to format it for the device." But as Phil explains, Google still promotes the use of "separate mobile sitemaps but also separate mobile sites." Which steers webmasters into an issue of having duplicate content. I agree with Phil, Google should really speak up on this matter.
In short, you can always do user agent detection and force mobile users to m.domain.com, while forcing others to www.domain.com, including GoogleBot (using a 301 redirect). You can also make sure GoogleBot does not index the mobile version by disallowing it in your robots.txt file. JohnMu of Google lists out many duplicate content documents to help webmasters with the issue, plus we have covered duplicate content issues over and over again.
- Deftly dealing with duplicate content
- Duplicate content due to scrapers
- Demystifying the "duplicate content penalty"
- Duplicate content summit at SMX Advanced
- Google, duplicate content caused by URL parameters, and you
Forum discussion at Google Webmaster Help.