js hangman with repeated characters

1var secretWord = 'apple',
2    letters    = '';
3
4function play(letter) {
5  if(letters.indexOf(letter) != -1) {
6    console.log("You've already tried '" + letter + "' ...");
7  }
8  else {
9    letters += letter;
10
11    if(secretWord.indexOf(letter) == -1) {
12      console.log(letter + ' -> wrong!');
13    }
14    else {
15      var progress = secretWord.replace(new RegExp('[^' + letters + ']', 'g'), '_');
16
17      console.log(letter + ' -> ' + progress);
18      
19      if(progress == secretWord) {
20        console.log('Well done!');
21      }
22    }
23  }
24}
25
26play('i');
27play('e');
28play('p');
29play('e');
30play('a');
31play('l');