Two versions of pages showing as indexed by Google - how remove duplicates?

[wesmozart]

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I have a site which has dropped down rankings significantly in Google in recent weeks. In looking around for clues to the cause I noticed something about the site which does not apply to any of my other sites - namely www. and non www. versions of the pages are showing up as indexed in a site:mydomain search.
I get results for most of the pages with a site:www.mysite search but many of the pages also show in a site:mysite search - so I guess these are technically duplicates in the eyes of Google.
Should I try to get the non www pages de-indexed and if so how do I go about it?
I have already set Google Webmaster Tools up to use the www version as the preferred domain. I think the two versions have come about due to something to do with the fact my domains are addon domains in cpanel of my primary account main domain and are created with the www....namely mysite.maindomain.
If I go into Webmaster Tools and set things to delete the non www version of the indexed pages ....is there a risk of the www version of the site being deleted too?
Any feedback on this matter would be greatly appreciated.

[dc000000]

If you fix the redeirct from one to the other then only one version will actually still exist - so the other version will eventually drop out of the index. It's not exactly an instant fix but I think this is what you need to do.
Have a word with your host if you are in any doubt to make sure this is set up correctly.

[wesmozart]

Thanks for your swift reply - I'll contact my host. I've just been reading some Google help guidelines and it seems to suggest the fact I've set the 'preferred domain' to the www version in Webmaster Tools will eventually lead to the non-www version of pages dropping out of the index.

[ambi4681]

Alternatively use the new-ish Canonical link attribute – a method of reducing duplicate content. Watch the video from Matt Cutts (Google) explaining all about it.

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