This year is unique as they’ll be autumn exams for every subject and if you get a higher grade in your autumn exam than you get this summer then you can get a new certificate showing your higher grade. They won’t know about your first attempt and if it’s lower you just don’t get a certificate with the new grade. I don’t know what employer would care, if you pass you’ve got a gcse, only do the retakes if you need a specific grade for some reason.