How to make a loop inside async def aiogram?

Question

Python

Good day to all! I am writing a telegram bot (aiogram). It is necessary that the id of new users be added to the text.txt file.
Everything would be fine, but here I ran into a problem. I don't need users to "duplicate" so I added a check if there is already a user with the same id, but soon realized that loops don't work inside async def. Tell me, please, what to do, and how can everything be implemented without a cycle?

import random
import logging
from aiogram import Bot, Dispatcher, executor, types
logging.basicConfig(level=logging.INFO)
bot = Bot(token="...")
dp=Dispatcher(bot)
@dp.message_handler(commands=['start', 'help'])
async def send_welcome(message: types.Message):
    await message.reply("Hi!\nI'm EchoBot!\nPowered by aiogram.")


@dp.message_handler()
async def echo(message: types.Message):
    f = open('users.txt', 'a+')
    isuser = False
    length = len(f.read())
    f.seek(0)
    user = types.User.get_current()
    f.seek(0)
    while (f.tell()<length):
        if (f.read(len(str(user.id))+4)=='id: '+str(user.id)):
           isuser = True
        while (f.read(1)!='\n'):
            print(f.read(1))

    if (isuser ==False):
        f.write('id: '+ str(user.id) +'; name: '+str (user.full_name) +'; username: ' +str(user.username)+';\n')
    f.close()
    await message.answer(message.text)



if __name__ == '__main__':
    executor.start_polling(dp, skip_updates=True)

As I already understood, cycles are simply skipped when executing the code.
To be honest, I have no idea how to implement a check without a loop, and I would be very grateful if you could help me!

UPD: Now I thought that it might be worth writing a separate program that will simply clear the same positions in the list. How good is this idea?

Answer 1

Problem solved. Loops work great, just

(f.tell() < length)

did not work correctly, so the cycle was skipped.
Now the code looks like this and works great:

import random
import logging
from aiogram import Bot, Dispatcher, executor, types
logging.basicConfig(level=logging.INFO)
bot = Bot(token="...")
dp=Dispatcher(bot)
@dp.message_handler(commands=['start', 'help'])
async def send_welcome(message: types.Message):
    await message.reply("Hi!\nI'm EchoBot!\nPowered by aiogram.")


@dp.message_handler()
async def echo(message: types.Message):
    f = open('users.txt', 'a+')
    isuser = False
    length = len(f.read())
    f.seek(0)
    user = types.User.get_current()
    f.seek(0)
    i=0
    while (f.read(1)):
        f.seek(f.tell()-1)
        print(f.tell())
        if (f.read(len(str(user.id))+4)=='id: '+str(user.id)):
           isuser = True
           break
        while(f.read(1)!='\n'):
            print('+' + str(f.tell()))



    if (isuser == False):
        f.write('id: '+ str(user.id) +'; name: '+str (user.full_name) +'; username: ' +str(user.username)+';\n')
    f.close()
    await message.answer(message.text)



if __name__ == '__main__':
    executor.start_polling(dp, skip_updates=True)