Let's try this problem to help us remember how to double a number.
Double 9.
To double a number, we multiply the number by 2.
When we add the same number more than once, we can think of it as multiplication, as we see in this video.
Write this addition as a multiplication. 4+4+4+4
If we are adding groups of the same size, we can also write it as a multiplication:
\text{Number of groups}\times\text{Amount in each group}