I have found that some of my F2C created articles are appearing twice in my filtered article lists. It turns out that this is because of the way we use joomla Contacts, which on our site are role-based rather than individual-based. A user (the article's author in this case) can be linked to more than one 'contact' if they have multiple roles. This results in multiple records from the join on the content to the contact_details database tables - hence the duplicates.
With the standard joomla category blog, the articles are listed just once as you would expect - the query selects just one (the latest) contact when creating the author link.
Not sure if you count this as a bug or not, but thought it worth posting in case others see duplicate entries in their lists.
Questions and answers for Form2Content, a Joomla CCK.