If you wonder, how the largest duplicated content is created? Through external resources or internal resources? You will be surprised to read this article, because biggest duplicated content is not external, but internal. Without a doubt lots of top ranking blogs and sites have to deal with other people using your content. To minimize your internal content duplication there are few facts you got to deal with, rather get rid of to build a firm foundation to establish your site as a canonical source.
Internal duplicated content might be created through various manners, such as the key page elements. The second comes from the content it self. If you have a web shop, with products and this might cause duplicated content. For example if you sell sunglasses and if you use the same description for every sunglass only the type is different. Then this might cause duplicated content. You see this with sites, which are automatically generated by an affiliate company. The best is to write original content for every different product you are selling. So if every page of your site is unique you have better chance ranking higher in the search engines.
If you write a blog, try using different title tag for every post. If you repeat the same title tag once in awhile, then this might cause duplicated content. Even the H1 tags can not prevent this matter, the search engines spiders will see this as a same page and the same meta tags on every page can not convince the search engines that the pages contain unique content. If you have a site and you have a snippet on your side bar, which contains products, press release or a blog, although the sidebars are secondary page elements, but the spiders might 'crawl' and see these page elements as duplicated content, because this does not contains additional info.
It is best to find out which pages generate the most traffic and those pages that doesn't generate any traffic should be replaced with more unique content and see if these pages generates more traffic in the future.