Actually my point of the thread is not about duplicate content, due to my experience and the clients I have done consultation for that blame their previous link building company about their drop in ranking, after doing some deep analysation, you found that majority of the sites are not fit to rank let alone stayed there when there is any google updates.
And this particular site, just happen to have internal duplicate content, and this site has always stayed rank for each updates, until I implement something to the site I shouldn't have, after removing the culprits the rankings return. My point is for website owner to check their site, before blaming Google or their link building company whenever they drop in rankings.
Also have always believe duplicate content was a myth, not until I dealth with over 40 local clients with duplicate content how badly it effected their site, you might be ranking right now with duplicate contents, good for you, but take it from someone who's has experience in dealing with this, and has several clients for the duplicate content case study, not just something I picked out of the blue.