SOURCE

var arr = [1,2,2,3,4,4,5,5,5,6,6,6,6];
function onlyOne(arr) {
  var brr = [];
  var crr = [];
  var index = null;
  for(i=0;i<arr.length;i++){
		if(brr.indexOf(arr[i]) == -1){
      brr.push(arr[i]);
    }else{
      if(crr.indexOf(arr[i]) == -1){
        crr.push(arr[i]);
      }
    }
  }
  for(i=0;i<crr.length;i++){
    index = brr.indexOf(crr[i]);
    brr.splice(index,1);
  }
  return brr;
}

console.log(onlyOne(arr));