This latest advice from Google is useful in that it clarifies Google’s position, which I quickly paraphrase below:
no duplicate content penalty Google rewards UNIQUENESS and the signals associated with ADDED VALUE Google FILTERS duplicate content.
Duplicate content can slow Google down in finding new content XML sitemaps are just about the BEST technical method of helping Google discover your new content.
Duplicate content is probably not going to set your marketing on Google wants you to concentrate signals in canonical documents, and it wants you to focus on making these canonical pages BETTER for USERS.
For SEO, its not necessarily the abundance of duplicate content on a website that is the real issue. It’s the lack of positive signals that NO unique content or added value provides that will fail to help you rank faster and better in Google.