Check unique input data to submit

<form name="f1" id="f1" onsubmit="return checkToSubmit();" method="post">
  <input type="text" name="i[]" value="i1" /><br />
  <input type="text" name="i[]" value="i2" /><br />
  <input type="text" name="i[]" value="i3" /><br />
  <input type="text" name="i[]" value="i4" /><br />
  <input type="text" name="i[]" value="i5" /><br />
  <input type="text" name="i[]" value="i6" /><br />
  <input type="submit" value="OK" />
</form>

<script>
function checkToSubmit() {
  var valid = true;
  var hasData = false;
  // storage unique values user entered
  var enterList = '';
  var inputs = document.f1['i[]'];
  var nInput = inputs.length;
  var vInput;
  for (var i = 0; i < nInput; i++) {
    var input = inputs[i];
    vInput = inputs[i].value;
    vInput = vInput.replace(/[^a-z0-9_-]/gi, '');//normalize data
    
    if (vInput == '') {
      continue;
    }
    hasData = true;
    
    if (enterList.indexOf(vInput.concat(', ')) != -1) {
      valid = false;
      alert('Already exist');
      input.focus();
      break;
    }
    
    enterList = enterList.concat(vInput.concat(', '));
  }
  
  if (!hasData) {
    valid = false;
    alert('No data to submit');
  }
  
  return valid;
}
</script>