The negative externalities of a plane ticket absolutely do not double the cost of a plane ticket. They are not significant - certainly smaller than the taxes already levied on flights.

What this tells you is that flights, as they exist, are a net positive even including negative carbon externalities. Neither banning these flights nor making them absurdly expensive is a net benefit for society.

When we consider the marginal negative environmental externality compared to rail travel, it is even smaller or perhaps negative.