Another common reason is that the CMS does not use nice clean URLs, but instead uses URLs like ?id=1&cat=2, where ID refers to the article and cat refers to the category. The URLs ?cat=2&id=1will render the same in most website systems, but will be completely different to search engines.
Comments Pagination
In WordPress, as well as some other systems, there is an option to paginate your comments. This results in content being duplicated in the post URL, article URL + comment-page-1, comment-page-2etc.
Printable page
If your content management system creates print-friendly mexico mobile database pages and you link to those pages from your article pages, Google will generally find them unless you specifically block them. Now, ask yourself: which version do you want Google to show? The one with your ad and surrounding content, or the one with just your article?
vs non-
This is one of the oldest in the book, but sometimes search engines still get it wrong: vs. non- duplicate content, when both versions of your site are accessible. Another less common situation, but one I’ve also seen, is vs. S duplicate content, where the same content is served through both.
"Canonical" URL
As we've seen, the fact that multiple URLs point to the same content is a problem, but it's solvable. People who work at a publication will usually be able to tell you easily what the "correct" URL for a particular article should be, but sometimes when you ask three people at the same company, you'll get three different answers...