There used to be a lot of discussion on whether to block both the www and the non-www versions of a URL, essentially inserting two versions. Anyone have comments on what should be blocked here in 2015?
You don't block it, you redirect one to the other. Up to you which one you want to use. For me, on a new site, I always go non-www to keep it shorter. For an existing site, I use whichever is indexed better in Google.