On a practical level I would contact all the schools that I was intending to apply to and make them aware of the situation.
This would include a letter from the exam board confirmsing the issue and a letter from the school acknowledging the error.
Then retake.
I would be suprised if you suffered rejection due to this error if you followed that. Unil you have spoken to them all, then you wont know what their stance is and they always have discretion on who to admit.