Buttons appear twice after ajax submission of form containing a file field.

This is not a bug in Wysiwyg module, but a consequence of how #ajax works, and there's nothing we can do about it in Wysiwyg.

The #ajax functionality is not intended to update the entire form, only parts of it. Or rather, it's not intended to actually replace the <form> element itself. At the least, I can't find an example of where that's actually done in a module. The examples.module always uses a different element in the 'wrapper' key than the form itself.

Replacing the form element itself means the internal reference to the entire form in ajax.js line 408 is no longer valid and is a lingering reference to the form that was before it got replaced by the insert command a few lines above.

Because Wysiwyg received orders to detach itself from the document before the elements were replaced, that version of the form no longer has the "wysiwyg-processed" class on the select box for picking the format, just like the new version of the form won't have.

The "insert" AJAX command triggers Drupal.attachBehaviors() on the inserted content (usually parts of the form) to make sure editors and other behaviors remain functional after AJAX updates. That prompts Wysiwyg to insert the editor like it normally would. To be sure all parts of the form remain functional ajax.js, just after line 408, will trigger Drupal.attachBehaviors() on the stored reference to the entire form after all AJAX commands have been executed.

That old form reference gets handed down to Wysiwyg as the context in which to [re]attach editor instances to any format selection box which, as noted earlier, no longer has the "wysiwyg-processed" class. The editor will always fetch a "fresh" reference to the current document before attaching itself, so it gets added to the new version of the form created by the "insert" AJAX command. Thus, there will be two editor instances rendered for the same field. The button duplication happens because the editor settings passed back with the AJAX command will in this case be merged with the existing settings for this field/format, causing button duplication in the toolbar and a few other oddities.

Working around all this is pretty easy by adding a div container inside the form element and replacing that instead.

function testform_form($form, $form_state) {
  return array(
    'simplewrapper' => array(
      '#type' => 'container',
      '#prefix' => '<div id="simplewrapper_div">',
      '#suffix' => '</div>',
      'name' => array(
        '#type' => 'textfield',
        '#title' => t('Name'),
        '#required' => TRUE,
      ),
      'resume' => array(
        '#type' => 'managed_file',
        '#title' => t('Resume'),
      ),
      'about' => array(
        '#type' => 'text_format',
        '#title' => t('About'),
        '#default_value' => '',
        '#format' => 'full_html'
      ),
      'submit' => array(
        '#type' => 'submit',
        '#value' => t('Submit'),
        '#submit' => array('testform_submit'),
        '#ajax' => array(
          'callback' => 'testform_ajax_submit',
          'event' => 'click',
          'wrapper' => 'simplewrapper_div',
        ),
      ),
    ),
  );
}

function testform_submit($form, &$form_state) {
  $form_state['msg'] = 'Thanks for submitting.';
}

function testform_ajax_submit($form, $form_state) {
  $errors = form_get_errors();
  if (count($errors) == 0) {
    return '<div>' . $form_state['msg'] . '</div>';
  }
  else {
    return $form['simplewrapper']; // Only return the wrapper
  }
}