hangman doesn't end after guessing the word

[vinci]

Hi,

I should say from the beginning that I'm very new in Python. I've been following a video tutorial where hangman is being tested. I've got the following code which seems to work for the guy who teaches, but not to me. And I simply don't understand the logic of it to be honest and can't understand how it can actually work.
This is the code:

import random
    # make a list of words
words = [
    'apple',
    'banana',
    'orange',
    'coconut',
    'strawberry',
    'lime',
    'grapefruit',
    'lemon',
    'kumquat',
    'blueberry',
    'melon'
]
while True:
    start = input("Press enter/return to start, or enter Q to quit")
    if start.lower() == 'q':
        break
    # pick a random word
    # choice functions picks a random item out of an iterable
    secret_word = random.choice(words)
    print("secret_word is {}".format(secret_word))
    bad_guesses = []
    good_guesses = []

    while len(bad_guesses) < 7 and len(good_guesses) != len(list(secret_word)):
    # draw spaces
    # draw guessed letters and strikes
        for letter in secret_word:
            if letter in good_guesses:
                print(letter, end='')
            else:
                print('_', end='')
        print('')
        print('Strikes: {}/7'.format(len(bad_guesses)))
        print('')

        # take a guess
        guess = input("Guess a letter: ").lower()

        if len(guess) != 1:
            print("You can only guess a single letter!")
            continue
        elif guess in bad_guesses or guess in good_guesses:
            print("You've already guessed that letter!")
            continue
        elif not guess.isalpha():
            print("You can only guess letters!")
            continue

        if guess in secret_word:
            good_guesses.append(guess)
            if len(good_guesses) == len(list(secret_word)):
                print("You win! The word was {}".format(secret_word))
                break
        else:
            bad_guesses.append(guess)
    else:
        print("You didn't guess it! My secret word was {}".format(secret_word))

What I don't understand is why how this line "if len(good_guesses) == len(list(secret_word)" can actually work. The only situation where I'd assume it can work is when you've only guessed words whose letters don't repeat. So for a word like 'blueberry' it won't work.
Is this a mistake that the guy has made?
How could I improve the code, so that the game ends also when I have to guess words whose letters repeat.

Thanks!