why does the unit of time enter twice in the unit of acceleration?

Because the speed has formula of distance divide time = m/s

and the acceleration is speed / time so (m/s) / s = m / (s*s) = m/s^2

hope this is understandably sure easy for you

Because acceleration is acquired through integration of velocity, in which it appears already once. Also, you have to take into account that acceleration measures the variation of speed (m/s, so time is already present), through time (again), which checks that time appears twice.