As I know Google can interpret duplicating content as search engine SPAM and as a result your site will be penalized and removed from index. If design of your site is changed frequently, it can be not good for your site ranking as well. It is wise to have a single version of the web site which accessible for search engine robots and deny indexing of other versions.
Information about robots.txt file you can find there
http://www.robotstxt.org/wc/faq.html
You can use meta-tag “ROBOTS” with content "NOINDEX, NOFOLLOW" to disable indexing of exact pages. You can read about it on the site specified abow.