Anytime you plant a mango tree, from a mono-embryonic seed, you end up with something unique. "a new variety " but to be considered worthy of being a named cultivar, really depends if it has any unique merits.
there is nothing wrong with this, it's how many varieties we know of today, came into existence, a chance seedling, that is somewhat improved over the parent(s). it might be 1 in 1000 but it happens.