change item in column based on the item before[solved]

[amdi40]

Hello I would like to be able to compare a number with another number, and if the numbers are equal to each other, then I would like to add a number to the last number. Example:

[Var]
1
1
3
4
5
I would first compare lines 1 and 2 and as they are equal to each other then I would add a number to line 2 fx 1. Then I would like to compare lines 2 and 3, and as they are going to be different(line 2 = 2 now) then the variable should not be changed

I have tried doing it like this:

df = pd.read_csv("newfile1.csv", dtype=str)
n = 2
with open('newfile1.csv', 'r') as csv_file:
    csv_reader = csv.DictReader(csv_file)
    with open('newfile2.csv', 'w') as new_file:
        fieldnames = ['datetime', 'intensity']
        csv_writer = csv.DictWriter(new_file, fieldnames=fieldnames, delimiter=',')
        csv_writer.writeheader()
        for line in csv_reader:
            for i in range(len('datetime') - 1):
                if 'datetime'[i + 1] == 'datetime'[i]:
                    'datetime'[i + 1] = 'datetime'[i] + pd.DateOffset(minutes=n)
            csv_writer.writerow(line)

The datetime variable that i would like to change does however not change
Looking forward to hearing from you
Kind regards
Amdi