doble, noun:

twice as much

El doble is a very useful word when you want to make comparisons.

Comes el doble que yo.
You eat twice as much as I do.

Notice how you use que in the previous and the next examples, and how you use yo and tú

Trabaja el doble que tú.
He works twice as hard as you do.

But when you’re comparing nouns, you use de:

Su sueldo es el doble del mió.
His salary’s twice as much as mine.