It's not twice the fuel. It takes most of the fuel to get to speed. At that point the booster is SIGNIFICANTLY lighter, so it takes (again with that word) SIGNIFICANTLY LESS fuel to slow down, and then to land.
However, you still have to remember that most of the fuel is used to lift the weight of the fuel.
Take the weight of the fuel required for the landing (plus the weight of the part of the rocket that contains that fuel that had to be added).
Now figure out how much additional fuel will be required to lift that. How much will the additional fuel (and additional rocket weight to contain the additional fuel) weigh? How much additional fuel will be required for that? And so on? And so on? It adds up.