Dialog multiple instantiations
Looks OK to me. jQueryUI will take care of most of the low-level stuff like not letting you open several dialogs and such (by simply not letting you click "behind" the modal).

The code could be cleaned up though:


Store $(item) in a variable (same with the modal element)
No need to quote those option names
Most of the options you're setting are already the default options
Extract repeated code into functions
Don't use plain attributes to store "jobid" and certainly not "value" (since that's a real attribute for some elements). It's invalid markup. Use data-* attributes instead.
Don't use the onclick attribute either. You have jQuery: Use it.
Consider using a more semantically appropriate element for the close-job button. Like &lt;a&gt; or &lt;button&gt;. A div is semantically void; it doesn't mean anything. An a or a button, meanwhile, implies that it's clickable and will perform some sort of a action.


JavaScript

$('.close-job').on('click', function (event) {
  var button = $(this),
      job = button.data('job'),
      uri = button.data('uri'),
      modal = $('#closeWarningModal');

  function closeJob() {
    // ...
  }

  function dismissModal() {
    modal.dialog('close');
  }

  if(modal.length) {
    modal.dialog({
      modal: true,
      title: 'Close Job',
      buttons: {
        Cancel: dismissModal,
        Close: function () {
          closeJob();
          dismissModal();
        } // &lt;-- there was an extra comma here; some versions of IE will choke on that
      }
    });
  } else {
    closeJob();
  }
});


HTML

&lt;button class="close-job" data-uri="closeurl" data-job="jobid"&gt;Close&lt;/button&gt;