So, what exactly is duplicate content? It's actually a fairly self-explanatory term. Duplicate content is any content duplicated in one or more pages on your website, or several instances of identical content on the same page.
In the beginning, it was a relatively well-known SEO trick to "fragment" (spam) pieces of content onto separate pages to give the search engine robots even more pages to look at. The increased number of pages on your website in turn increased your chances of ranking well for multiple terms. That trick doesn't work so well today, however.
Google, in particular, has become very sensitive to duplicate content. If you are considering using content in various places on your website, you should be aware that Google appears to have implemented several duplication filters within its algorithm. These filters have been known to place penalties on websites that use excessive duplicate content to increase their chances of obtaining better rankings.
If you are using a database-driven website or a website that uses a template system, there are some things you can do in order to reduce the risk of suffering from these penalties.
- For databases, be sure your pages are accessible through only one single URL (i.e. multiple instances of the same page do not exist).
Optimally, each page should have unique title and meta tags, since these pieces of information are more intensively used to determine rankings.
As a general rule, any two webpages on your website should be at least 75% unique, which is to say that for every 100 sentences of words on any given webpage, at least 75 of them should be unique to that page. So, there's some room to have instructions, information, or other content that needs to be present on every page.
This rule also applies across domains. So, if you have "borrowed" content from other websites and their pages were on the Internet first, chances are that Google (and other search engines) may not display your page in search results for some search phrases, since it might be considered a duplicate page of another website. Again, having at least 75% unique content will ensure that you don't become penalized by a duplication filter, so just borrowing a single phrase of even a paragraph shouldn't be too much of a problem.