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I had mydomain.com, my domain.net, mydomain.org and mydomain.co.uk all pointing to the same site, and at one stage Google had multiple copies of most of the pages in its index.
Recently, I've set up 301 redirects on the net, org, and co.uk domains to the com domain, and Google is now just listing that site, with increased PR :-)
I finally had the site optimized and was doing well, then *zap* I'm out of my keyword. (I tweeked myself out of that keyword as I later discovered)
So I started tracking hits to the site and separating the hits from each domain, and sure enough I am getting about 25% of my SE bots hitting my old domain. I even have a link to the old domain from a recip. link site where I know I put in the new domain.
I still don't know if having both domains hit the same IP is a bad thing, but I set up a new 'homepage' for the old domain which then transfers people to the other domain. I have new plans for this old domain now, (an info site concerning the target industry as a whole) so after I have my content organized I will have two sites on one IP (linked together of course) and more content than the site has ever had. Bonus for me and the SE's!