Web content and search engine optimization techniques mostly revolve on how original or unique your content are in light of search engine policies concerning duplicate content. To be more specific, search quality depends on the uniqueness of your site, although duplicate content in and of itself isn't something that's penalized, despite possible belief. Even on Google's Webmaster Guidelines on duplicate content outright acknowledges that most duplicate content has been made without deception in mind, so Google has no intention of penalizing each and every last instance of it.
The thing is, because plagiarizers, spammers, and scammers typically use duplicate and redundant content, people have begun associating it as a punishable offense, when in fact using it for the sake of spamming is the real reason why sites like Google penalize people for using it. If you want further information concerning duplicate content and Google's take on it, simply Google the Webmaster Guidelines on duplicate content.
The Great Duplicate Content Myth
The existence of duplicate content penalties is a myth. To reiterate, what Google actually penalizes are sites that spam using duplicate content, not sites that use duplicate content for a perfectly reasonable goal (like, say, a printable version of the same web content). Many webmasters who don't know better worry that Google penalizes sites for merely having duplicate content in them; this myth is further perpetuated by the query message that says that in order for it to show the most relevant results, some sites with duplicate content has been omitted.
There are some who repeat the search and see that their sites have been omitted from Google's SERP (search engine results page), which leads them to conclude that they've been penalized. There has been evidence of people being worried about this as being considered a "strike" for the "infraction" of providing duplicate content. What's really occurring here is that Google wants diversity in their search results, and they can't achieve that by letting a popular site's duplicate content fill the entirety of the SERP. They want to deliver unique and highly relevant content with each and every search query.
Differences between Spam and Non-Spam Duplicate Content
If for example someone searches for the "My Little Pony: Friendship is Magic" television series on Google, the search engine will typically provide a search result of the Wikipedia article or the main Hasbro page on that particular cartoon. However, the printable versions of those pages that showcase the exact same content will be omitted to avoid redundancy. There isn't really any penalty from the omission though, because if you care to for the print articles for the latest My Little Pony series, the omission would be reversed and you'll end up with all the printable versions of the websites while the web versions get removed from the SERP.
This issue is handled on a case-by-case, per-query basis more than anything else. However, there are exceptions to this rule, such that there will be penalties for those who abuse the use of duplicate content for more nefarious purposes. More to the point, the ones being penalized are typically spammers that use duplicate content for spamming. The penalty is for the spam, not the presence of duplicate content per se. Distinction between web results, uniqueness of content, and a strict policy against spam is what helped Google become a leader in the search engine field, after all.
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