ahah fails on 2nd attempt with 2 elements on page

i have a small form function which i have 2 copies of on a page. at the moment the form is simply an ahah submit button that flips the text of the button back and forth.

the first of these seems to flip back and forth fine as many times as i like.

the second button flips the first time; but the second time the input field has lost the ahah-processed class and when i click the button is submits the page (no ahah).

the code in its entirety is:

function ajax_signup_button_form($form_state, $nid) {
  ahah_helper_register($form, $form_state); 
    
  global $user;
  $signed = false;
  
  $signups = signup_get_signups($nid);
  foreach ($signups as $signup) {
    if($signup->uid == $user->uid) {
      $signed = true;
      break;
    }
  }
  
  // handle button being pressed
  if (isset($form_state['values'])) {
    if ($form_state['values']['signup'] == "Cancel Alert") {
      //signup_cancel_signup($signup, false);
      $signed = false;
    }
    if ($form_state['values']['signup'] == "Alert Me!!") {
      //signup_cancel_signup($signup, false);
      $signed = true;
    }    
    
  }

  if (!$signed) {
    $value = t('Alert Me!!');
  }
  else {
    $value = t('Cancel Alert'); 
  } 
  
  $form['signup-button']['signup'] = array(
    '#type' => 'submit',
    '#value' => $value,
    '#name' => 'signup',
    '#ahah' => array(
      'event' => 'click',
      'path'    => ahah_helper_path(array('signup-button')), 
      'wrapper' => 'submit-button-' . $nid,
      'method' => 'replace',
      'effect' => 'fade',
     ),
    '#prefix' => '<div id="submit-button-' . $nid . '">',
    '#suffix' => '</div>',
    '#tree' => true,
  );
  
  return $form;  
}

not entirely sure if this is meant to work with Submit buttons but it seems to work quite well except for this issue; i also do not require any submit handler; i simply process the button click in the same function (as you can see in my code).