Modified TrafficCop for jQuery 1.5+: http://freshbrewedcode.com/jimcowart/2011/11/25/traffic-cop/

jquery.trafficcop.js

/*
    TrafficCop
    Author: Jim Cowart
    License: Dual licensed MIT (http://www.opensource.org/licenses/mit-license) & GPL (http://www.opensource.org/licenses/gpl-license)
    Version 0.1.0
*/
(function($, undefined) {

var inProgress = {};

$.trafficCop = function(url, options) {
    var reqOptions = url, key;
    if(arguments.length === 2) {
        reqOptions = $.extend(true, options, { url: url });
    }
    key = JSON.stringify(reqOptions);
    if (key in inProgress) {
        for (i in {success: 1, error: 1, complete: 1}) {
            inProgress[key][i](reqOptions[i]);
        }
    } else {
        inProgress[key] = $.ajax(reqOptions).always(function () { delete inProgress[key]; })
    }
    return inProgress[key];
};

})(jQuery);