Duplicate content is an issue that every site owner should be concerned about. Some things that cause duplicate content can be dealt with easier than others.
For instance. If your site can be accessed by:
domain.com/index.html (or whatever the extension)
That is 4 different URLs that point to the same content on one web page. That is duplicate (quadruple) content.
The above example is easy to avoid. Pick one way to access your site. Then use a 301 redirect. direct domain.com to www.domain.com or www.domain.com to domain.com. I always redirect to www.domain.com because in my experience most people that link to your site naturally will use the www version.
Always link to your home page from internal pages using an absolute link. Never link back using the relative link index.html (or whatever your extension is).
Sometimes these issues never cause a problem. Sometimes they do. It is better to do things right from the beginning than wait for trouble to develop, and then try to correct the problem.
Eventually the SEs will pick one page and drop the others. In the mean time these duplicate pages can split PR. Some people refer to a duplicate content penalty. I don’t think there is normally a penalty for duplicate content. The SEs have filters to try and determine duplicate content, and filter out some pages.
To be continued.
Nearly all blogs and forums have duplicate content issues. Many e-commerce sites, as well as sites that build pages dynamically have duplicate content problems. Why? because you can access any page (or content) from several different URLs. Unfortunately addressing these issues is a little more difficult than what I have referred to above.
Then you have to consider affiliate sites, and scraper sites.
But the dumbest of all duplicate content issues is when a webmaster builds two completely different sites with similar content. In this case you might get a penalty.