Canonicalisation is a very long word that can be causing issues within your website and you may not be aware of it. When you register a domain name you are registering the domain without any additional parts such as www. or mail. These are in reality a sub-domain of the domain name.
Canonicalisation typically can be seen with a website when it can be fully navigated as both the www version of the domain name and also the non-www version of the domain. The search engine effectively has the ability to create 2 version of the same site, seeing them as being different (or actually a duplication of the content). Again I’ll suggest you take a look at what Matt Cutts has to say from a post way back in 2006.
It may not be causing an issue but it only take a few links to your non-www version of your website and the process can start. Google are doing a lot of work around assisting with removing canonical issues within a website by providing tools within the Google Webmasters Toolset to help you identify issues within your websites beyond just the non-www and www versions, caused by e-commerce / CMS solutions.
I would suggest you check the domain for the site and also whilst your doing this, make sure that any additional domain names pointing into your website also get pointed correctly to the right domain, as again this will cause issues with duplication.
If you think you have a problem then contact me and I can check to see if this is the case and look at working with you to put the correct types of redirects in place to remove the duplication.
