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Internet Marketing Guides » SEO Tips » Put the Kaibosh on Duplicate Content
With regard to duplicate content, this can happen a number of ways. The most nefarious way that this takes place and can hinder your results in SEO is via multiple URL's generated from a CMS that all direct to the exact same content. This method can double or even triple the copies of content on the site. This relates to using HTTP VS HTTPS or if its www.filipecsantos.com VS filipecsantos.com or filipecsantos.com/index.html, which are all the same page! The search engine isn't smart enough yet to discern this (or perhaps they do not want to because other variations may exist at those URL's depending on configuration), so we must control what is in our realm or deal with the consequences of our inaction.
Next, we have near duplicate content which can be the effect of a dynamic generated page from a CMS or perhaps a page you were trying to rank for similar terms using most of the same content. There is also one other way that comes to mind where this takes place, and many times without you even aware of it - print pages. While print pages usually strip out navigational elements, the content is going to be verbatim and can be a problem with search engines for two reasons, 1) it can be seen as a useless double if linked to normally and 2) it is likely what's known as a "leaf page".
"leaf pages" are pages that are one offs and have no navigation, so if one landed into the page from a search engine for example, they would have no option out into the rest of the site but to type in the domain or remove the URL segments from the address bar of their browser (which people generally aren't going to do). For search engine spiders - this matters a lot, because if a user cannot navigate past the page, likely a spider is going to be stuck there too and likely leave the site without indexing the site properly - potentially skipping over your most important content.
Content that is duplicate and externalized (on someone else's site) is bad, but not going to harm your SEO efforts all that much (most of the time) and is certainly not as bad as having duplicate content within your own website. As the owner of the site, you have control versus other's websites - so addressing these issues is totally within your grasp. One issue that relates to you as a website owner or SEO for a specific online property (IE: a client), is if you or your client own multiple URL's that are not redirected properly to the primary website URL, rather are duplicates of the exact same architecture, site navigation and content. This is a BIG NO NO, but I see it all of the time. Avoid this and ensure that you are 301 redirecting all iterative or multiple domains owned.