Duplicate content is identical or almost identical content found on the same website or other websites. Ideally, Google wants to feature only one version of the same content and will usually select the oldest, most authoritative domains, dropping less authoritative domains with content penalties. Duplicate content can also hurt or prevent rankings for original content. It can hurt sales. In this article we discuss duplicate content issues in detail.
Duplicate content generally refers to substantive blocks of content within or across domains that either completely match other content or are appreciably similar. Mostly, this is not deceptive in origin. As a result, the ranking of the site may suffer, or the site might be removed entirely from the Google index, in which case it will no longer appear in Google's search results.
Affiliate Sites
Google can spot affiliate footprints such as Amazon and eBay code. Once spotted, Google filters out affiliate websites from search results, leaving the most authoritative domain. The goal is to show only one version of content to users in search results, hence one website is enough.
Affiliate sites may also promote products without the feed, but with an affiliate link. Search engines are also aware of those links and may filter out websites with low trust scores.
Online Merchants
Online retailers are hit primarily because their descriptions are:
100% scraped from manufacturers and are identical to dozens of competitors.
Too similar, or too short, with little difference from page to page.
If you run a retail site, you have to invest time or money in creating descriptions that are different from the ones provided by the manufacturer(s) and from competing websites that use the same manufacturer descriptions. About 150 - 300 words of original content is usually enough to differentiate yourself in the eyes of search robots.
Search Optimization 
