[Python] How can I stop my tkinter hangman game accepting a correct letter more than once?

I am making a python hangman game using tkinter, however, after guessing a correct letter, it continues to accept that letter but marking it as wrong. This is my guess function:

def guess_letter(self, event=None):
guess = self.entry.get().lower()
self.entry.delete(0, tk.END)

if not guess or len(guess) != 1 or not guess.isalpha()
return

self.guessed_letters.add(guess)
self.guessed_label.config(text="Guessed Letters: " + ", ".join(sorted(self.guessed_letters)))

if guess in self.word:
for i, letter in enumerate(self.word):
if letter == guess:
self.display_word = guess
self.word_label.config(text=" ".join(self.display_word))
else:
self.tries_left -= 1
self.draw_hangman()

if "_" not in self.display_word:
messagebox.showinfo("Hangman", "You won!")
self.master.quit()
elif self.tries_left == 0:
messagebox.showinfo("Hangman", f"You lost! The word was: {''.join(self.word)}")
self.master.quit()


I want it only to register guesses as wrong if they have not been guessed before. What is a method I could use to make it not accept an already guessed letter without any repercussions?

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