Sys.Application.add_load(ApplicationLoadHandler)
function ApplicationLoadHandler(sender, args) {
    if (!Sys.WebForms.PageRequestManager.getInstance().get_isInAsyncPostBack()) {
        Sys.WebForms.PageRequestManager.getInstance().add_initializeRequest(InitializeRequest);
    }
}

var lastPostBackElement;
function InitializeRequest(sender, args) {
    var prm = Sys.WebForms.PageRequestManager.getInstance();
    if (prm.get_isInAsyncPostBack() && args.get_postBackElement().id === lastPostBackElement) {
        args.set_cancel(true);            
    }
    lastPostBackElement = args.get_postBackElement().id;
}

if (typeof (Sys) !== "undefined") Sys.Application.notifyScriptLoaded();