what is dual degree in BTech ina university please explain

So, basically Dual degree with respect to B.tech means that you will be getting your B.Tech anand M. Tech degree separately.You will get a B. Tech Degree after 4 year course and then a M. Tech Degree after 2 year course.
What you will get with dual degree is the fees of M.Tech will be same when you started your B Tech degree initially.
There is also a integrated course in which you can do B.tech+M.tech in 5 years.There are many universities in India that offer these courses.You can checkout the names of the colleges from the link below :
https://engineering.careers360.com/colleges/list-of-dual-degree-engineering-colleges-in-india