siteground add-on domains and duplicate content issues

Bluehost's approach to add-on domains is that you have one main domain and all additional domains are subdomains of the first.

I am concerned about the SEO implications of this, since it creates duplicate content.

The same files can be accessed at:

second.first.com
first.com/second
second.com

I've set up htaccess redirects to take care of the problem, but I'd rather avoid the problem completely when I create demo sites for clients.

So, I'm thinking of going with siteground.

Two different Siteground reps gave me two different answers. The second one insisted that add-on domains can't be accessed as domains or folders of the main site.

Does anyone happen to know if this is true?

For hosts like Bluehost, what are the best ways to take care of the problem to steer clear of SEO issues with duplicate content?

I'm thinking of getting a separate account just for client demo sites which I'll not tell search engines about and maybe make only private Wordpress sites on it.

Is that enough security, or are there other steps I should take to protect client sites?

For example, should I have a robots.txt file and disallow everything?

Thank so much!

Laura