CAPTCHA disappears after AJAX submission

I've been experiencing a problem where the CAPTCHA on a form disappears after an ajax submission is triggered for the form by an addressfield widget. I select a new country, there's an ajax submission to update the addressfield fields for that country, and the CAPTCHA test disappears.

I've traced the cause of this to the fact that the captcha element's #process callback is getting called twice during the ajax submission (probably indicating that the form is getting rebuilt twice as well) and so what happens is this: The javascript settings added inside recaptcha_captca() get added twice, and they get merged by Drupal with array_merge_recursive() inside ajax_render(). That results in Drupal.settings.recaptcha containing this:

{
  theme: ["clean", "clean"],
  lang: ["en", "en"],
  public_key: ["....", "...."]
  container: ["recaptcha_ajax_api_container", "recaptcha_ajax_api_container"]
}

... instead of this:

{
  theme: "clean",
  lang: "en",
  public_key: "....",
  container: "recaptcha_ajax_api_container"
}

Obviously this breaks things. I have resolved this by using a static variable to ensure that the drupal_add_js() calls only happen once per captcha session per request. I'll provide a patch for this.