If we grow mongthong from seed will that tree also give same mongthong fruit?
No, if you plant durian from seed, the fruit of the resulting tree are not the same as the parent tree. But you might get lucky, our tastiest tree is a Monthong planted from seed, we sell all the "real" Monthongs and eat the fruit from that one ourselves
Also, at least around here, people like to plant a few Chanee trees among Monthong to help with pollination, so it's quite possible that there's some Chanee heritage in the Monthong seeds...