How to stop Telegram bot executing old code

I made my first simple telegram bot with python-telegram-bot. Here's the code:

from telegram.ext import Updater, CommandHandler, MessageHandler, Filters
token = '1234567890'
updater = Updater(token=token, use_context=True)
dispatcher = updater.dispatcher
def start(update, context):
    context.bot.send_message(chat_id = update.effective_chat.id, text=f"hey,{update.effective_chat.username}!")
    
            
def unknown(update, context):
    context.bot.send_message(chat_id=update.effective_chat.id, text="Sorry, didn't understand")

unknown_handler = MessageHandler(Filters.command, unknown)
    
start_handler = CommandHandler('start', start)
dispatcher.add_handler(start_handler)
dispatcher.add_handler(unknown_handler)
updater.start_polling()

I kept changing the message text in the code and then executing it, checking what happens in the bot. What I ended up is every time I do start, the bot sends any of the old message versions together with new one. I revoked the token and it helped.

My question is what happens under the hood and why the latest version of the code doesn't replace the old ones? Also, how can I deal with it without having to revoke token every time?