In a FAF Thon event, a participant can only be registered once. In order for the participant to be registered twice he (or she) must have two separate contact records in the database.
If the records have already been merged, follow this Alternative Solution:
  1. Create a Sphere-only registration type that has no fee associated
  2. Re-register the participant through Bookkeeping > Single Registration Entry, creating a NEW contact record
    • Be sure that this new record has different contact information from the original record so that the two will not be merged again based on your auto-dedupe settings