Are You Duping And Don't Know ItGreat talk here by Rand from SEOmoz about what constitutes duplicate content and how to avoid the black mark having dupe content can create with Google. We all need to duplicate content some time.
Rand explains how to grab a piece of something and create enough unique content around it that Google considers the mashup unique. He also explains programming ideas such as canonicalization of your urls. Play by Rand's Rules to avoid content duplication and you won't lose points with the Google Gods.