duplicate site for testing purpouses without being penalized

If I duplicate my site for new design, will SE find me and penalize me?

[silverbytes]

Scenario:
Good ranked site is under redesign: mysite.com
Alternative domain is mysite.com.something,
The fear is about being penalized for duplicating contents in diferent domains and spoil the good rank of main site.

Alternative Site is just for testing redesign, it will not be submited to SE.

I wonder if thereīs a safe method of preventing that alternative site with duplicated content to being seen by SE...

What would be the best options?
Iīve thought:

robots.txt file like:
User-Agent: *
Disallow: /*

Is this sintax valid? copying and pasting this into notepad and uploading to root would be enough?

In the other hand.. will alternative site be found if I never submit it? Does that risk really exists?

[skipfactor]

There are many methods. Personally, on new pages that I don't want indexed for the time being, I add:

<META name="robots" content="NOINDEX, NOFOLLOW">

Then when I'm ready for the page(s) to be indexed, I do a "Find & Replace", replacing with:

<META name="robots" content="INDEX, FOLLOW">

[Mark_A]

if on apache why not keep it behind an htaccess file and only let in the test users with their own passwords?

[silverbytes]

About using meta tag: I donīt think that would be a good idea on this case. I should put that code on each page of a big site (some 90 htmls).

About apache: how do I do that, would you mind to explain?

And could someone please tell me about the robots.txt method?

[skipfactor]

robots.txt Tutorial [searchengineworld.com]