If the recipe calls for 3/4 cup of sugar and it is doubled, you would multiply the amount of sugar by 2.
\[ \text{Amount of sugar needed} = 2 \times \frac{3}{4} = \frac{6}{4} = \frac{3}{2} \text{ or } 1 \frac{1}{2} \text{ cups} \]
Therefore, if the recipe is doubled, you will need 1 1/2 cups of sugar.