1. Dublicated content will get you in truble with some major SE's.
2. If your first site is ranked low, Why dublicate the content that gave the low ranking. Change the content around, try optimaizating the content and pages. Just changing the adress woun'd do any good
As for moving a site, I had the same prob. One of my hobby sites was on a www.mysite.hostcompany.co.uk url, and I wanted to replace it with www.mysite.co.uk that I'd set up.
I dropped a .htaccess redirect 301 onto the old location, removed all the old content from there, copied the content to the new hosting package I'd bought with the new .co.uk domain pointing to it, and sat back to see what happened.
A few months down the road now, it seems ok. The old url dropped out of the SERPs and the new url (same page content etc) replaced it.
I had thought that having a proper .co.uk domain name with the domain name being very relevant to the subject *might* even boost it up a bit, but I didn't notice any change.
I think so long as you don't have the same content sat in both locations at the same time, you shouldn't get hammered for having dupe content.