Export participants includes duplicate records when a registered participant has been merged with another contact. The attached CSV file is from the Drupal demo.
I do not know which of these are essential conditions, but these conditions duplicate the error:
1. Create a contact that includes First and Last Names, email address and is set to "Use Household Address" so that it has address info as well.
2. Create a "no fee" event with Online Registration that is publicly accessible and only requires first, last and email.
3. Without logging in to the site, register for the event using a similar but different last name: Freddy instead of Fred for example, the same last name and same email. This creates a new contact with this info in the db.
4. In the Admin section, search for matching contacts and merge the events & activity from the sparse contact to the fully addressed contact.
5. Return to CiviEvent, show participants for the event, select the newly merged contact and a couple others and export. The merged contact will be duplicated in the CSV file.