1st Don't rush into anything, You must decide the end goal and results you would like to achieve.
2nd Duplicate content is bad, although you may not get a penalty it is definitely a black mark.
3rd 301's are a good option like Aaron said, just bear in mind you will likely lose the position in the search engine for the old URL and you may not regain the position with the new domain. I would recommend using a url rewrite in the htaccess file otherwise you will end up with a generic 301 and a lot of links broken.
Let's say this is some of your old URLs:-
www.domain.com/blog/how-to-look-smart
Let's say your new site is www.newdomain.com (With the same content links etc), If you did a standard 301 it would divert to www.newdomain.com However the subsequent URL after the.com would be lost i.e. they would just end up on the homepage of www.newdomain.com not www.newdomain.com/product/shoes.
In your htaccess file you set up a rewrite so it would redirect to the new domain name without changing the URL structure like this www.domain.com/product/shoes Redirects to www.newdomain.com/product/shoes

