Google’s thought of Content Duplication

Duplicate contents across the same same websites or different domains are not indexed by Google, Adam Lasnik says at the Offcial Google blog. Google’s algorithm prevents such sites to be indexed and such sites may be removed from googlebots crawl list. Translation in different languages is different. Such sites content may be filtered from the SERPs. Tips given are :-

Webmasters should use either the robot.txt or use meta tag index nofollow to stop googlebot from crawling such pages. TLD’s should be used.

Suppose We are building a web directory or we run a website where user submits contents or comments from other websites, which are duplicate, or for example on a forum where there is user participation, What’s the webmaster’s job in this?

Google should be intelligent to understand this?