Will I be penalized if I mirror my site example.com with example.ca? There's slight differences but essentially the same.
The fear for duplicate content are often exaggerated. Nevertheless, try to make those sites as different as possible. I'm sure you can find a few unique points for each of the sites.
i think using 301 redirect from one to other is a better option because if you mirror them, search engines may consider them as duplicate content
Yes you will penalize. use some trick change menu name, its position, sequence and style. Rephrase the content, try to provide some thing additional.