When building your website to grow and grow over time you need to focus on content, now more than ever. Duplicate content can mean several different things: content that is the same between several pages on your own website, content that is similar to content on other websites, and more recently content which isn’t seen as being useful to visitors because better or more authoritative versions of it can be found on other sites. Avoiding duplicate content is becoming more and more important as Google continues to expand what it views as duplicate content. If you think you have duplicate content on your site it is best to remove it or combine it so that you do not get punished(have your websites results buried below other results or worst case scenario, even be removed from getting indexed in search results), however if you have too much duplicate content and have been punished you may not be able to recover.
Google looks at duplicate content for many reasons, the most important is to give searches different information. If Google sees the same information in more than one place they pick a version of the content top display and the other gets buried because when searching they want to display different information about the same search. Goggle’s Panda update punished many sites for having duplicate content, which started to include sites that were directories, or sites that aggregated information from other sites. Citysearch.com has seen a massive decrease in traffic since Google panda. On the other hand, yelp has seen a continual growth owing to not just being a directory of places, but a site which focuses on useful reviews, something which Google encourages.
What happened to city search also shows the danger of thin content, that is a site that has content not copied from other sources, but a site whose content only covers the most basic of element. Having only a small amount of information, so that visitors cant find everything they are looking for and have to leave to other webspaces leads to being punished by Google. The more robust your content, the more value you give to a user coming to your website, the more people are interacting with your content, the better your site is seen Google, and you will continue to see your website rise in the search results.
Think of it as overall added value to the web in general, if your site gives people new information, or combines information in interesting ways, or is helpful to people than you will be rewarded. On the other hand if you are trying to trick your way into ranking up, or are just doing the same thing as others you will have the opposite happen.
Duplicate content is all bad and does not have to be your largest concern, but large amounts of duplicate content both within sites and between sites can signal that you are a spam company to Google and get all your sites punished. To avoid punishment Google lays out some simple rules to follow which can be found here.