Prevent multiple request on scroll

I use this code to get data from server while scrolling using jQuery AJAX:

 var pageIndex = 0;
 var pageCount;

    $(window).scroll(function () {

        if (($(window).scrollTop()) == ($(document).height() - $(window).height())) {
            GetRecords();
        }
    });
    $(function () {
        GetRecords();
    });
    function GetRecords() {
        pageIndex++;
        if (pageIndex == 1 || pageIndex <= pageCount) {
            $("#loader").show();
            $.ajax({
                type: "POST",
                url: "http://ift.tt/1Q1wtQq",
                data: '{pageIndex: ' + pageIndex + '}',
                contentType: "application/json; charset=utf-8",
                dataType: "json",
                success: OnSuccess,
                failure: function (response) {
                    alert(response.responseText);
                },
                error: function (response) {
                    alert(response.responseText);
                }
            });
        }
    }

But : 1- how can I prevent multiple request to webmethod at a moment? 2- when I add a toolbar to my browser (below of address bar) scroll method doesn't work any more.