Mistake when sends many ajax in same time

Screen lock not removed sometimes when ajax response returned. It's because in same time can be send other ajax, which set unblock=false and not runed blockUI().
First simple solution, check if screen lock already showed. For this we should use condition and check onblock variable.

Here my example, which contain other my patches:

(function($) {
  var unblock;
  $(document).ajaxSend(function(event, jqxhr, settings) {
    if (unblock != true) {
      unblock = false;
      var target = settings.url;
      var pages = Drupal.settings.ajax_screen_lock.pages;
      var visibility = Drupal.settings.ajax_screen_lock.visibility;
      var throbber_hide = Drupal.settings.ajax_screen_lock.throbber_hide;
      var disabled = Drupal.settings.ajax_screen_lock.disabled;
      var blockui = function () {
        unblock = true;
        if (throbber_hide) {
          $('.ajax-progress-throbber').hide();
        }

        $.blockUI({
          message: Drupal.settings.ajax_screen_lock.message,
          css: {
            top: ($(window).height() - 400) / 2 + 'px',
            left: ($(window).width() - 400) / 2 + 'px',
            width: '400px'
          },
          timeout: Drupal.settings.ajax_screen_lock.timeout
        });
      }

      // If is not disabled.
      if (!disabled) {
        if (!$.isEmptyObject(pages)) {
          // Handle pages.
          $.each(pages, function (num, page) {
            if (target.length >= page.trim().length) {
              if (target.substr(0, page.trim().length) == page.trim() && visibility == 1) {
                blockui();
              }
              else if (visibility == 0 && target.substr(0, page.trim().length) != page.trim()) {
                blockui();
              }
            }
          });
        }
        else {
          blockui();
        }
      }
    }
  });

  $(document).ajaxStop(function(r, s) {
    if (unblock) {
      $.unblockUI();
    }
  });
}(jQuery));