Hello Aspirant,
See Passing certificate is issued only once and if you improve your marks later on through On Demand Examinations then you will get new marksheet with updated marks but only of that subject for which you appear for ODE and then you can apply for passing certificate as well for those 2 updated subjects, note that this will only be valid in the case if you had appeared for NIOS exams in your first attempt.
I hope this information helps you.
Good Luck!!
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