The calendar event module does not allow you to change the name of attendees. Instead, do the following to add the correct 
attendee and remove the duplicate ticket:

First, do an offline refund of the duplicate ticket, following these steps:
  1. Go to the event list
  2. Click Manage
  3. Look up the person with the duplicate ticket.
  4. Click Refund.
  5. At the bottom of the refund screen, you will see an option to refund to the original credit card or "No, the donor has already been reimbursed or will be reimbursed offline. Just record the refund in the Luminate Online database." Choose the second option. 
Second, register the correct participant from the back end, recording an off-line payment. 
  1. Go to the event list
  2. Click Manage
  3. Click Add Event Attendee
  4. Fill in fields with correct attendee information
  5. For payment method, choose cash. This ensures that no money is actually charged for the replacement registration. 
You can discourage this behavior by adding a logout link log-out link to the last page of the registration. This will prevent the form from auto-filling when the user returns to register another participant.