Duplicated panes above the Account pane.

If there are any other panes above the Account pane on the same checkout pages, they will be duplicated, because commerce_checkout_login_account_form uses the same $form array as was used in pane callback in commerce_checkout_form():

...
if ($callback = commerce_checkout_pane_callback($checkout_pane, 'checkout_form')) {
      // Generate the pane form.
      $pane_form = $callback($form, $form_state, $checkout_pane, $order);
 ...

We need to use $form = array() in commerce_checkout_login_account_form() to not include previous elements from the checkout page form.