What happens if I resit and get a lower grade? Watch
If you re-sit a whole qualification then you will get a second certificate and have both results. Which one counts depends on the person looking at them - i.e. if this is for a university place you'd have to ask the university. Over time you can probably 'forget' the lower result for CVs.
If you're re-sitting a unit before getting an overall grade then different exam boards have different rules but generally the better result counts.