What would happen if the moon doubled in mass

Technically, the Earth would now be the Moon’s satellite, since its mass would become smaller.

Tides on Earth would be higher, and by a lot, even by dozens of times.

These large tidal forces would greatly increase the planet’s volcanic activity, to a level probably comparable to that of the moons of gaseous planets, and it would cause the Earth to overheat internally.

Therefore, in addition to huge tides and giant volcanoes, our planet would start to resemble something like this:

At the same time, the numerous volcanoes would also have the effect of blocking sunlight, thus cooling Earth’s surface.