Ok! I have a fix! I have replaced:

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

With:

        if not guess or len(guess) != 1 or not guess.isalpha() or guess in self.guessed_letters:
            return

This checks if a letter has already been guessed, and doesn't take away lives if it has! ^_^