Question

why does the unite of time enter twice in the unite of acceleration

Answer 1

Acceleration is velocity per unit time, and velocity is displacement per unit time. In other terms, velocity is change in displacement per unit time, and acceleration is change in velocity per unit time.

The reason acceleration is in units of [distance]/[time]^2 is because it is really [distance]/[time]/[time] or how the velocity is changing per unit time.