Duplicate Activity History records are created and displayed in Activity History selector when a user registers online for an event.
To recreate on demo:
- Go to event registration (preview mode is fine): http://demo.civicrm.org/drupal/civicrm/event/register?id=2&reset=1&action=preview
- Now go to Activities tab for that contact and click Activity History
- You'll see that there are two entries in the selector for the same activity_id (but their id= values are different).