Understanding SEO issues related to Duplicate Content

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Duplicate content is very common on the web. It comes in the forms of content syndication, mirror sites, content scraping, quoting, reusing content, data fed sites etc.

Duplicate content is a challenge for the search engines for at least the following 3 reasons:

Duplicate content drains search engine resources in the form of disk space, CPU power and manpower, which could be used for the processing of other content
When users submit a query to a search engine, most users do not want links to (and descriptions of) web pages that have duplicate information
Duplicate content can be used by web spammers and content thieves

There are essentially 2 types of duplicate content - duplicate documents and query-specific duplicate documents.
Duplicate Documents

Duplicate documents are web pages that are the same or almost the same. The duplication here operates on the whole page, unlike…
Query-Specific Duplicate Documents

Sometimes documents can be quite different on the whole, but share common paragraphs. Examples:

one document quoting paragraphs of another document
two pages data fed the same product information but having a different navigation around the duplicate text
link directories can have common website listings.