Is a Mobile Website Considered Duplicate Content?

There is a debate in the mobile SEO world small village:

Is having a mobile version of a website considered duplicate content?

No.

Matt Cutts, known as “Google’s SEO guy”, addresses this issue in a nice and quick video.

Basically, Google is smart enough to tell whether a website is meant for mobile or desktop. This means Google keeps track in its index of sites, the fact that you have a separate mobile version. So if you have a mobile version of your website, don’t worry about duplicate content.

This is also stated in a more confusing way on the Official Google Webmaster Central blog:

Once Googlebot-Mobile crawls your URLs, we then check for whether the URL is viewable on a mobile device. Pages we determine aren’t viewable on a mobile phone won’t be included in our mobile site index (although they may be included in the regular web index).

Does Your Mobile Site Need a Different URL?

No.

Google’s goal is to offer desktop searchers desktop sites, and mobile users mobile sites. A desktop user won’t see your mobile site come up in a search for your keywords.

In the above video, Cutts offers one suggestion for mobile site owners. Creating a separate URL for your mobile site – m.yoursite.com – can be helpful for testing purposes, or to help visitors understand they are inside a different version of your main site. But in terms of SEO, it’s not necessary.

Further reading:
Google Webmaster Central Blog – “Help Google Index Your Mobile Site”
Search Engine Land – “Don’t Penalize Yourself: Mobile Sites are Not Duplicate Content”