Hello student,

YES you can do class 11th and 12th again from the UP board even if you have already passed class 12th from the NIOS board and it is legal to do that.

Students who have passed the secondary/senior secondary course from any National/state board of school education and want to take admission for the same course, you can continue your education towards a Senior Secondary Certification, equivalent to XII standard.

I hope that this will help.