Hi, I have seen article directories like ezinearticles.com, articlesbase.com contains TONs of duplicate content. I too submit single articles to dozens of articles directories and all get published. Now why Google not penalize such sites? noticeably all article directories are having high PR.
PageRank has little to do with duplicate content. PageRank is all about who links to you. Those long lasting popular article directories attract a ton of natural links, especially from seo blogs & forums. So they have a lot of trust built up with Google. As for the duplicate content, Google understands that there is legitimate duplicate content. Google cares more about content that gets duplicated on more than one page of your site. Also these article directories are usually smart enough to add something to the title, meta description and body content so that overall it seems unique.
It could also be that Google considers that ezine article as being the original source. People tend to spend the most time promoting their ezine articles most of all.
PageRank has nothing to do with duplicate content. PageRank is about the quantity and quality of backlinks to the site.