Hey, if a candidate wants to repeat 12th to improve his/her marks they can do so. It will not affect the future of the candidate in any way, in fact, improvement exam will help in the future if the first marks were too bad and the improvement results are good. They will only look at the marks so if you are worried that sitting for an improvement exam will be bad for the future then don't. it will hardly make any difference other than your marks

Best wishes