hey there,

Yes , It is legal.

But as you said you are already pass and if  you are not satisfied with your marks then you can fill improvement examination form

Or you want to repeat your 11th and 12th with different stream then yes it is legal and you can repeat your class again by same board or different board.

hope this helps you

Yes,you can do so.

But let me raise a question to you why do you want to do so. If you have already passed CBSE board. Most of the people anwser is to increase my percentage. My dear friend how many time should i tell you Guy's that it's just paper of shit nothing more than that if you will graduate from a college noone is going to ask you for 12th marksheet. Just wastage of time as well as wastage of money. Do you evenly imagine that what all you can do in a single year. A lot productive work can be done in a single year than to pursue again 12th. I don't understand why you people can't understand such a small thing.