Getting new visa without using the previous one?

I have a question. I want to apply a 60 day tourist one in my own country. but I heard sometimes it is not possible to get one and they just give you a 30 day one. If this happens I can also try to get a student type to stay longer. Is this possible if i already have a 30 day one which I have not used. Will they automatically cancel the 30 day one?