It doesn't matter really, as long as you don't have both, as this is a canonical error. A lot of webmasters find that they have duplicate content errors when they see an SEO report on their websites, and aren't sure why. It's because their website comes up as both www and non-www, essentially duplicating the content.
It's really easy to find out if this is effecting your site by just typing your website with www, then without the www into a browser. After pressing enter and going to your site, the URL on the address bar should only show it one way. Which means if you type your website into your browser with www and it shows up as www, then type it in without the www and it shows up without the www, then you have a problem. Basically, your site's URL should only appear 1 way after you actually go to the site regardless if you type it in with or without the www.
If your site is coming up as both, then a quick 301 redirect from the non-www to the www (or vice versa) will fix it.
If you've already done link building to the site, then it would be more efficient to have your site the way you've built your links. For example, if you built your links with www in the URL, then that's what you should redirect your site to.
Hope that helps!