Often we talk about duplicate content when large portions of text are identical copies of other written on different sites and that Google considers spam, is not quite as desired, for example, in forums where they are with post with duplicate documents see for example the tag often used to reattach a response to a given question lies perhaps two pages back.

More often the spider carefully evaluate whether this is intentional or not, trying not to harm anyone by assessing attention duplicate pages.

Some examples of websites and then duplicates could be considered spam:

* Websites identity, identical pages, these are certainly considered as SPAM
* Stolen content, content is copied verbatim to the new site and resold as new
* E-commerce websites, which often reproduce products with technical data copied from the manufacturer
* Items copied in various blogs and proposed

A useful tool to find copied content is called Copyscape

Is there a way to be sure that the spiders do not read two pages in your site that is indentical to properly configure the robots.txt file to prevent access to one of the two pages so as to direct it to the correct page.