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Looping through Json object
$(document).ready(function(){
    $.getJSON('XML.php', function(data) {
        JSON.stringify(data);
        var prevCardCode = '';

        $.each( data, function(index, element){
            var prevCardCode = element['CardCode'];

            if (!(element['CardCode'] == prevCardCode)) {

                var sum = element['payment_sum'] + '<br/>';
                $('#showdata').append(sum);
                }
                alert(element['CardCode'] + 'compare' + prevCardCode);
        });

    });
});

var prevCardCode = '';
$.each(data, function(index, element) {
    if (!(element['CardCode'] == prevCardCode)) {
        var sum = element['payment_sum'] + '<br/>';
        $('#showdata').append(sum);
    }
    alert(element['CardCode'] + 'compare' + prevCardCode);
    prevCardCode = element['CardCode'];
});

var prevCardCode = '';
$.each( data, function(index, element) {
    if (!(element['CardCode'] == prevCardCode)) {
        // your code...
    }

    prevCardCode = element['CardCode'];
});

var i = 1;
var precedentEl = array[0];

while (i < array.length) {

   // Do some stuff like compare current and precedent

   precedentEl = array[i];
   i++;
}