If you’ve ever wondered what Google does and doesn’t clasify as ‘duplicate content’ then you might find this explanation of it on their official Webmaster blog.
A few snippets:
“What is duplicate content?
Duplicate content generally refers to substantive blocks of content within or across domains that either completely match other content or are appreciably similar. Most of the time when we see this, it’s unintentional or at least not malicious in origin: forums that generate both regular and stripped-down mobile-targeted pages, store items shown (and — worse yet — linked) via multiple distinct URLs, and so on. In some cases, content is duplicated across domains in an attempt to manipulate search engine rankings or garner more traffic via popular or long-tail queries….
What does Google do about it?
During our crawling and when serving search results, we try hard to index and show pages with distinct information. This filtering means, for instance, that if your site has articles in “regular” and “printer” versions and neither set is blocked in robots.txt or via a noindex meta tag, we’ll choose one version to list. In the rare cases in which we perceive that duplicate content may be shown with intent to manipulate our rankings and deceive our users, we’ll also make appropriate adjustments in the indexing and ranking of the sites involved. However, we prefer to focus on filtering rather than ranking adjustments … so in the vast majority of cases, the worst thing that’ll befall webmasters is to see the “less desired” version of a page shown in our index….
Don’t fret too much about sites that scrape (misappropriate and republish) your content. Though annoying, it’s highly unlikely that such sites can negatively impact your site’s presence in Google. If you do spot a case that’s particularly frustrating, you are welcome to file a DMCA request to claim ownership of the content and have us deal with the rogue site.”
This last part will undoubtedly calm a few nervous bloggers that I know who worry about their content being scraped and republished on other people’s blogs and being penalized for it.