I assume that its easier for more massive planets to have a dense and massive atmosphere, and that the less massive a planet gets the more likely the gas will escape into space.
Is earth sufficiently massive to hold on to an atmosphere with double the amounts of gases in our atmosphere? If it were possible would that be the equivalent of going in a hyperbaric chamber like they have for some types of medical treatments?
Different question. If a string of ice comets totaling about half the mass of the moon had collided with the early earth formation when most of the surface was molten, what would that much more water do? Would the current time frame surface of the earth be completely covered with oceans?