1. If the participant is already a registered to the event:
Either remove the gift from the batch
OR
Delete the participant record from the event prior to committing batch

2. If the participant was registered for the event twice, and the second participant record was already deleted from the event but the error is still showing in the batch:
a. In the gift batch, highlight the affected gift
b. From the menu bar, select Gifts> Links> Events> Registration Fees
c  Click Load Registrations From, search for and select the correct participant record. This will reload the registration options, and the registration that was deleted will no longer appear.
d. Click the link button and add the amount to apply to the registration, and click ok.

3. If the participant should have multiple registrations:
a. In Configuration, select Business Rules
b. Select Event Options
c. Mark the 'Allow multiple constituent registrations' checkbox in the 'When adding participants to an event' frame