Cname Domain Map - Duplicate content penalty in Google?

[triumph]

I would like to map my typepad site widgets.typepad.com to widgets.com. However, I can not do the recommended 301, because I have no control over Typepad's .HTAcesss and it doesn't support PHP. Typepad's domain mapping solution involves a CNAME change which points widgets.com -> widgets.typepad.com. As a result, both sites are served simultaneously. This is great for backlinks and maintenance of the site, but I'm afraid I'm going to get a duplicate content penalty from Google. I've seen other typepad sites use the domain map, and they seem to have survived, but I can't get any real data, since most bloggers don't monitor their serps. Any help would GREATLY appreciated, because I have to eventually migrate off typepad do to resource constraints, and the first step would be to map my own domain.