This issue can occur if a user registers for themselves and a guest account and selects their name in the Guest field.  To the Raiser's Edge it appears they are trying to register twice due to the settings of Config > Business rules.

To resolve this issue:
  1. Go to Config > Business Rules > Event Options.
  2. Mark the box next to "Allow multiple constituent registration"
  3. Once done, you can now process the Event Registration from Online Express again.