Calculus: Why do they antidifferentiate twice

I know the when you take the antiderivative of something like

4x is 2x^2+C

But then I see someone take the antiderivative twice without the +Cs.

Can someone explain why they take antiderivative more than once and when?
Ty.

It totally depends on the context of the problem why they are doing it, but a common problem would be to find f(x) from f''(x). For example, if s(t) is the position of a particle at time t, and a(t) is the acceleration of the particle, then a is the second derivative of s (velocity is the rate of change of position and acceleration is the rate of change of velocity). Therefore, you would need to integrate twice to find s(t) from a(t).

Another thing you might be seeing, and I don't know how far along in calculus you are, but they may have been integrating with respect to different variables. To integrate a function with 2 independent variables (i.e. f(x,y) = xy), you would take what's called the double integral. One integral is for x and the other is for y.