Google is quick to point out that "Duplicate content on a site is not grounds for action on that site unless it appears that the intent of the duplicate content is to be deceptive and manipulate search engine results." However, the algorithm is far from perfect, no matter how many tweaks Google's engineers are making, and you don't want to risk your site getting penalized for duplicate content, even if it wasn't malicious.Here are a few ways that Google recommends site owners can proactively avoid a duplicate content issue on their site:
Use 301s: One of the most common high-level SEO issues I see many sites have is that they have multiple homepages (usually mysite.com and mysite.com/index). This isn't usually done in an attempt to trick the search engines, but rather a common byproduct of when the site was created. This is a negative for your website for a couple of reasons. First off, having two homepages means the link juice is being split between the two. Most links are going to be sent to the .com page, but some of your internal links (like logos or the HOME tab in the high level navigation) might go to the .com/index homepage. This can devalue your real homepage and you might not get all the SEO value from the inbound and internal links that you should. Google is usually pretty good at determining which homepage they should index, but they know the algorithm isn't perfect and you don't want to risk a duplicate content issue. Use a 301 redirect to permanently move all links pointing to the .com/index page to the .com page (saving your SEO efforts) to make sure there is only one homepage.
Syndicate carefully: If you are using content pulled from another site, say you want to republish an interesting blog post on your own company blog, be sure to cite your source and give them a link back! Many of the Google Panda updates were designed to penalize sites that republished others' content and claimed it as their own. Some of these plagiarizers were actually outranking the original authors in the search results, which had a lot of site owners and content writers up in arms. There is nothing wrong with reposting someone else's content as long as you give them full credit!
Avoid publishing stubs: Google recommends that you avoid using placeholder pages since most visitors don't want to be directed to a blank page. If you don't have the content ready to go for a new page on your website, hold off on publishing until everything is ready to go. "If you do create placeholder pages, use the noindex meta tag to block these pages from being indexed."
Minimize similar content: Avoid having multiple pages on your website that publish almost identical information. If you find yourself repeating information time and time again, consolidate those pages into one really great and well-written page. Not only is this better for your visitors, because they no longer have to read the same page over and over, it also gives the search spiders better content to read and index.