Can you override single submit forms?
If you haven't already, try using button rather than submit type.

'#type' =&gt; 'button',



  When the button is pressed, the form will be submitted to Drupal,
  where it is validated and rebuilt. The submit handler is not invoked.


Perhaps this will prevent that formSingleSubmit behavior from running???



If the above doesn't work, since all the formSingleSumit behaviour does is prevent form from being submitted if it hasn't changed and nothing else. Therefore, you don't need this behaviour and since Drupal is global, you could just do in custom.js:

delete Drupal.behaviors.formSingleSubmit;


OR set it/override it to empty

Drupal.behaviors.formSingleSubmit = {};