The records are most likely merged due to the Real-Time Dedupe  (BB709998) settings within Sphere.

Through the Sphere interface your options are to leave both registrations or delete one of the registrations, preferably the most recent one.

To prevent the merge from occurring:
  1. Edit (BB709965) the Real Time Dedupe settings.
  2. Register in a way to bypass the dedupe settings: for example, if the real-time dedupe settings are first name, last name, and email address match exactly, alter one of these fields when registering.
  3. Create a new Kintera Thon event each year to prevent the merging of records.