How to check if value exists in JavaScript object in order not double?

You can use this. Use an array to check if the value has already read.

Use hotelArray.indexOf(val) == -1 to check if value exist in array.

var totalHotel = json.SearchAvailResponse.Hotel.length;
// console.log(totalHotel);
var hotel = '';
var hotelArray = [];
for (var i = 0; i < totalHotel; i++) {
  val = json.SearchAvailResponse.Hotel[i].Name;
  console.log(json.SearchAvailResponse.Hotel[i].Name);
  if (hotelArray.indexOf(val) == -1) {
    hotelArray.push(json.SearchAvailResponse.Hotel[i].Name);
  }
}

$('#hotel').html(hotelArray.join('<br/>'));

DEMO

var totalHotel = json.SearchAvailResponse.Hotel.length;
// console.log(totalHotel);
var hotel = '';
var hotels = [];
for(var i=0;i<totalHotel;i++){
        if(jQuery.inArray(json.SearchAvailResponse.Hotel[i].Name, hotels) == -1)
    {
      hotels.push(json.SearchAvailResponse.Hotel[i].Name);
      hotel += json.SearchAvailResponse.Hotel[i].Name;
      hotel += '<br/>';    
    }
}

Demo -> https://jsfiddle.net/83dbha0h/

You can use jQuery's each() to simplify the looping, and you can use object properties as a quick way to register what hotels have already been seen.

Here's the code to make it work:

var allHotels = json.SearchAvailResponse.Hotel;
var seenHotels = {};
$(allHotels).each(function(i, hotel) {
    if (!seenHotels[hotel.Name]) {
        $("#hotel").append("<p>" + hotel.Name + "</p>");
    }
    seenHotels[hotel.Name] = true;
});

You can see that in action in this Fiddle: https://jsfiddle.net/fzaey8jw/

first create an object (means a hashmap)

var hotels = json.SearchAvailResponse.Hotel.reduce(function(map, obj) {
    map[obj.Name] = obj.HCode;
    return map;
}, {});

after that you can do this:

var hotel = Object.keys(hotels).sort().join('<br/>');

You can do like this

var json = {
    "SearchAvailResponse": {
        "Hotel": [{
            "HCode": "IDJKT_00393",
            "Name": "All Seasons Jakarta Gajah Mada (Opening July 2013)",
        }, {
            "HCode": "IDJKT_00448",
            "Name": "Holiday Inn Express Jakarta Pluit"
        }, {
            "HCode": "IDJKT_00448",
            "Name": "Holiday Inn Express Jakarta Pluit"
        }, {
            "HCode": "IDJKT_00448",
            "Name": "Holiday Inn Express Jakarta Pluit"
        }, {
            "HCode": "IDJKT_00448",
            "Name": "Holiday Inn Express Jakarta Pluit"
        }, {
            "HCode": "IDJKT_00337",
            "Name": "Novotel Gajah Mada"
        }, {
            "HCode": "IDJKT_00337",
            "Name": "Novotel Gajah Mada"
        }, {
            "HCode": "IDJKT_00344",
            "Name": "Rota International"
        }]
    }
};
k={};
var totalHotel = json.SearchAvailResponse.Hotel.length;
for(i=0;i<totalHotel;i++){
  k[json.SearchAvailResponse.Hotel[i].Name]=0;
}
var hotel=Object.keys(k).join("<br/>");
$('#hotel').html(hotel);

Here I am making a object again but key of object will be name of Hotels since key can'not be duplicate in object so its taking only once next i am joining those key with and getting result. Hope it will Help

Have you tried by simply check if the name is already in your generated html?

var totalHotel = json.SearchAvailResponse.Hotel.length;
    // console.log(totalHotel);
    var hotel = '';
    for(var i=0;i<totalHotel;i++){
        console.log(json.SearchAvailResponse.Hotel[i].Name);
        var thisHotel = json.SearchAvailResponse.Hotel[i].Name;
        if(hotel.indexOf(thisHotel) === -1){
           hotel += thisHotel;
           hotel += '<br/>';
        }
    }

    $('#hotel').html(hotel);

this is working, in your jsfiddle the return is:

All Seasons Jakarta Gajah Mada (Opening July 2013)
Holiday Inn Express Jakarta Pluit
Novotel Gajah Mada
Rota International