One of the most common problems I see in websites is the same content being available at both the WWW and non-WWW versions of a domain. I’ve encountered this in nearly every website I’ve done an SEO audit for, and I see it every day when browsing the web. Despite it being so prevalent, it is indeed a problem.
Having the same content available on both the WWW and non-WWW versions of a domain (such as authoritylabs.com and www.authoritylabs.com) is called canonicalization. While you and I might realize they are in fact the same page, search engines mistake them to be unique pages.
Most of the time, search engines can figure out that they are the same page and only include the canonical URL in their index. SEObook explains the canonical URL as:
The canonical version of any URL is the single most authoritative version indexed by major search engines. Search engines typically use PageRank or a similar measure to determine which version of a URL is the canonical URL.
Regardless, canonicalization can result in indexing problems and duplicate content issues. Most importantly, canonicalization will split the link juice between each version as people link to and share both.
What you want to see is a redirection from the WWW to the non-WWW, or vice versa, so that if the wrong version is entered or linked to, the user is automatically taken to the canonical URL. Fortunately, this is relatively easy to set up.