Final answer:
The responsibility to confirm that the applicant does not have existing accident or sickness insurance lies with the insurer when applying for Medicare supplement insurance.
Step-by-step explanation:
When a person applies for Medicare supplement insurance, it is the responsibility of the insurer to confirm that the applicant does not already have accident or sickness insurance already in force. The insurer will typically review the applicant's medical history and inquire about any existing insurance coverage before approving the application.