Remove duplicate elements in array

In a sorted Array, remove the duplicate elements

[1,1,2,2,3,3,3,4] -> [1,2,3,4]

High level:

Fast: Being processed index

Slow: left of slow is kept

Case 1: f == 0, copy f++ s++

Case 2: a[f] = a[s - 1], dont want, f++

Case 3: