Duplicate content is not a complex subject as some would have you believe, it is simply defined by me as two (or more) pieces of the same content which appear on different pages on the Internet. In terms of internal duplicate content there are many reasons this can happen such as:
Duplicate content can confuse search engines in several ways;
This can have two implications to web masters 1) as search engines will rarely show multiple versions of the same content in search results they are forced to decide which version of the content is 'best'. This process could result in the version you consider best not be shown and the other versions of your content are essentially negated (their 'power' is not utilise). 2) if your duplicate content problem is large enough search engines could deem it worthy of a penalty or as Google put it in their webmaster post on the subject;
make appropriate adjustments in the indexing and ranking of the sites involved. As a result, the ranking of the site may suffer, or the site might be removed entirely from the Google index, in which case it will no longer appear in search results.
Luckily for you there are several ways for you to solve these problems but there are two which are particularly useful:
<link href="http://www.yourdomain.com/original-page/" rel="canonical" />
Although these are the two most common ways to deal with duplicate content the 'noindex, follow tag', preferred domain tool in Google Webmaster Tools and parameter handling tool in Google Webmaster Tools are also viable.
I hope this helps with your issues and if you have any questions let us know!