Out of the box, Drupal has powerful optimization features built into its architecture. But without care, it can easily generate duplicate content that you might not be aware of. The following are common problems that may arise.

• URL Aliases - being able to access the site with both the original url and the alias
• Page titles - having titles that are too similar or identical
• Accessing your site with and without the www
• Printer friendly pages
• Forums and Views that create sortable lists that can generate multiple URLs with duplicate content.
• Forward to friend links
• Allowing multiple terms per node resulting in the same teasers appearing on different category pages

Fortunately, all of these issues can be easily resolved with a couple additional modules and some simple changes to the robots.txt file and the .htaccess file.