In the land of SEO, there's only "One domain name to rule them all." Which means that its best practive that only one domain name points to your content. Since Google doesn't know the difference between visiting www.yoursite.com, yoursite.com, yoursite.net or yoursite.co.uk (or other domains you point to your site) you'll need to fix this, to avoid that Google assumes it's dealing with duplicate content.

This is sometimes called "canonicalization." The goal of this is to make all domains redirect to one domain, properly. Including the www. or non-www version of your main domain name.