Search engines index www and non-www URLs as different domains. This means that links to http://www.example.com and http://example.com are counted separately.
You can fix this by choosing which format you prefer (also called the canonical one) and redirecting the other one to it. You will need to use a server-side 301 redirect to redirect all your other domains to the preferred one. This is considered an advanced strategy and if done wrong can do real damage to your site. Here’s a complete step by step guide to be sure you get your 301 redirect right: How to Use a 301 Redirect to Keep Your Web Pages from Dropping Off the Face of the Earth.
Tip: If you doubt what domain should become your preferred, run our Backlink Checker and choose the one with the most backlinks.
Note: Be sure that all links on your site (including the Sitemap) point to the correct URLs. No links or Sitemap listings should point to a URL that redirects!