Python - Comparacion palabras

with open('users.txt', 'r') as f:

        users_dni = set() # guardo temporalmente los DNI

        users = []

        for line in f.readlines():

            dni = line.replace('\n','').split(',')[2].split(':')[1] # recupero el DNI de la linea actual

            if dni not in users_dni:

                users_dni.add(dni)

                users.append(line)

    with open('users_without_repeat.txt', 'w') as f:

        f.writelines(users)