Here is my experience with duplicate content.
First off, Google will not give you a penalty for duplicate content.
Google is not foolproof and sometimes duplicate content will rank, sometimes it can even outrank the original content. If Google recognizes the content as being duplicate, it will not rank it. In most cases, it will not even index the page. I have messed around with duplicate content on web 2.0's and other tier 2 links and even if I get it to index it is only a matter of time before it gets deindexed. Your content must be unique or you run the risk of never getting it to index or remain indexed.