I have another solution just use two pointers pattern and swap 2 elements in that array.

class Solution {

  public static int remove(int[] arr) {
    // TODO: Write your code here
    int n = arr.length;
    int first = 0;
    if (n < 2) return 1;
    int second = 1;
    while (first < n && second < n) {
      if (arr[first] != arr[second]) {
        if (second - first > 1) {
          int tmp = arr[first + 1];
          arr[first + 1] = arr[second];
          arr[second] = tmp;
        }
        first++;
        second++;
      }
      else {
        second++;
      }
    }
    return first + 1;
  }
}