Hi

Yes, you can have 6th or the additional or optional subject in your 10th or 12th.

And yes,

as one has to choose  subjects in 11th only for 11th and 12th  so you will have to study the optional subject in your both 11th and 12th, similarly for class 10th , now registration happens in 9th itself. So, you will have to study the optional subject in both 9th and 10th.

BENEFITS OF ADDITIONAL SUBJECT

Now,

coming to benefits of optional subject, the only benefit is that you get to learn and know one more subject and sometimes it can make you eligible for some exams as well related to your optional subject

For example,

if there is a student with the subject physics chemistry mathematics English and Hindi in 12th then he won't be eligible for Neet examination but if he has biology as an additional subject or the 6th or the optional subject then he will be eligible and will be able to attempt Neet .

Sometimes,

You even develop interest because of additional subject. For example, I had Hindustani music as  an additional subject in 11th and 12th , before studying it I had no idea about music, but in those two years of 11th and 12th I got to know  a lot about music and learned it as well and whatever I learnt will always remain with me :)

But ,

if you talk about benefit in government services because of your additional subject then sorry to tell you there will be no benefit due to the fact that you had additional subject in class 10th and 10 + 2

This is because,

Government services are provided through competitive examination and they don't see whether you have studied additional subject or not ,what matters is that you  just eligible for that particular competitive examination with your qualification and the rest i.e  your selection depends upon your performance in the competitive exam.

For example,

For those aspiring for government services, IAS is not less than a  dream but to be eligible for IAS , you need to be just a graduate in any discipline ,your subject doesn't matter,  and the rest depends upon your performance in civil services examination.

If you have any other doubt then you can always reach us to clear them :) we are always available to help you out and guide you

Thank you