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.
I used to do this with a small plug-in in Joomla 1.5 but the time of Joomla! 1.5 is far beyond is. So I needed to find an alternative. As it turns out, I found the solution in an extension I already install by default. Cue "Akeeba Admin Tools Professional".
1. First of all, make sure you've got Admin Tools PROFESSIONAL installed. If you're serious about websites, you already have an Akeeba Deluxe subscription anyway, so this should not be a problem.
2. Go to Components -> Admin Tools.
3. In Admin Tools, click ".htaccess maker.
4. Open Optimisation & Utilities. Select "redirect non-www to www" at 'Redirect www and non-www adresses'. This will make sure that visitors who try to go to yoursite.com will be redirected to www.yoursite.com.
5. In the field below, with the label 'Redirect this (old) domain name to the new one', type the secondary domains. Don't forget to add "temporary" domains that might be assigned by your hosting company, as well.
6. Click "Save & Create .htaccess. Admin Tools will now create a new .htaccess files which will handle the proper redirecting!
You're now done, but if you want to take it one step further, you can read the Akeeba Manuals and use the "SEO and link tools" option in Admin Tools.