Most webmasters worried about duplicate content penalty. But I think they are over worried. Duplicate content penalty is not what they really think.
If you are scraping content from other sites and putting them to your site without having additional utility to user, there may be a penalty.
Here are some occasions where duplicate penalty really occurs.
1. You are scrapping content from other sites. You do not give additional data. For example, you are scrapping data from Wikipedia and putting them as it is.
2. You have an affiliate. You display products and services of the main site without proving any additional data.
These practices should be avoided
• Don't create multiple pages, sub-domains, or domains with substantially duplicate content.
• Avoid... "cookie cutter" approaches such as affiliate programs with little or no original content.
You submit same article to 20 article directory sites with a link back to your site. Will it results duplicate content penalty?
This is a duplicate content situation. But Search engines like Google will not punish anyone. You can’t actually control the process which copies your article to multiple directories. Article directories cannot prevent people submitting duplicate articles. So Google will give top rank to the original article and give lesser rank for duplicate articles.
Check this comment on http://googlewebmastercentral.blogspot.com/
“Most search engines strive for a certain level of variety; they want to show you ten different results on a search results page, not ten different URLs that all have the same content. To this end, Google tries to filter out duplicate documents so that users experience less redundancy. You can find details in this blog post, which states:
1.When we detect duplicate content, such as through variations caused by URL parameters, we group the duplicate URLs into one cluster.
2.We select what we think is the "best" URL to represent the cluster in search results.
3.We then consolidate properties of the URLs in the cluster, such as link popularity, to the representative URL.”
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