No need to reappaear for all subjects. 

Just make sure yiur PCB aggregate scores in 12th above 60% for AIIMS MBBS eligibility . You can appear to any subject more than one or just one where you have scored least or have failed.

Your best of both marks will be considered for counselling. 

Eligibility criteria sharedbelow exam. 

https://medicine.careers360.com/articles/aiims-mbbs-eligibility-criteria