SEO is a very hush hush topic, understandably no one is going to share their knowledge outside what is generally considered common-knowledge. If you are serious about SEO my best advice is read up the general common knowledge, and then rip apart dozens to hundreds of competitors sites at core levels..
Compare and contrast many top placements for your Keywords -> niche -> market -> related markets, then once your full of knowledge, develope your content and linking structures around your knowledge.
SEO is a forever changing concept, if you are planning to ever achieve the top or stay on top, you must educate yourself very thoroughly of where search engine algorithms are, and where they are going
edit: sorry for not answering your question.. Duplicate content is a changing and evolving area, to best understand it, you must understand where search engines are going in the future. No one is going to be able to give you a very accurate density % or anything
