Let say you have this array
integers = [1,1]
for i in range(0, a_len) will be expanded to for i in [0,1]
the first time i = 0
when executing integers.count(1), it will return 2
then you delete from the integers array once, the array now is 1 element long.
In the second iteration i is equal to 1, but the numbers only have 1 element (you erased one),
trying to access its index 1 will throw an exception

arr_new.count(x) will alwasy be 0, because the array is empty.