no error but keep getting "letter has already been guessed. please try again"

public class Hangman {

    public static void main(String[] args) {
        // Enter amazing code here:
      Game game = new Game("treehouse");
      Prompter prompter = new Prompter(game);
      prompter.play();
    }
}

import java.io.Console;

public class Prompter {
  private Game mGame;

  public Prompter(Game game) {
    mGame = game;
  }

  public void play() {
    while (mGame.getRemainingTries() > 0) {
     displayProgress();
     promptForGuess();
    }
  }

  public boolean promptForGuess() {
    Console console = System.console();
    boolean isHit = false;
    boolean isValidGuess = false;
    while (!isValidGuess) {
      String guessAsString = console.readLine("Enter a letter: ");
      try {
        isHit = mGame.applyGuess(guessAsString);
        mGame.applyGuess(guessAsString);
        isValidGuess = true;
      } catch (IllegalArgumentException iae) {
       console.printf("%s. Please try again.\n", iae.getMessage());
      }
    }
   return isHit;
  }

  public void displayProgress() {
   System.out.printf("You have %d tries left to solve: %s\n",mGame.getRemainingTries(), mGame.getCurrentProgress());
  }


}

public class Game {
  public static final int MAX_MISSES = 7;
  private String mAnswer;
  private String mHits;
  private String mMisses;

  public Game(String answer) {
    mAnswer = answer;
    mHits = "";
    mMisses = "";
  }

  private char validateGuess(char letter) {
   if (!Character.isLetter(letter)) { 
     throw new IllegalArgumentException("A letter is required");
   }
    letter = Character.toLowerCase(letter);
    if (mMisses.indexOf(letter) >= 0 || mHits.indexOf(letter) >= 0) {
      throw new IllegalArgumentException(letter + " has already been guessed");
    }
    return letter;
  }

  public boolean applyGuess(String letters) {
    if (letters.length() == 0) {
      throw new IllegalArgumentException("no letter found");
    }
    return applyGuess(letters.charAt(0));
  }

  public boolean applyGuess(char letter) {
    letter = validateGuess(letter);
    boolean isHit = mAnswer.indexOf(letter) >= 0;
    if (isHit) {
      mHits += letter; 
    } else {
      mMisses += letter; 
    }
    return isHit;
}

  public String getCurrentProgress() {
    String progress = "";
    for (char letter: mAnswer.toCharArray()) {
     char display =  '-';
      if (mHits.indexOf(letter) >= 0) {
        display = letter;
      }
      progress += display;
    }
    return progress;
  }

  public int getRemainingTries () {
    return MAX_MISSES - mMisses.length();
  }

}