more and more modal-backdrop when open modal

Created by: beviz

I imitate the sample of docs to write the following code:

<a data-toggle="modal" href="/products" data-target="#modal">
  load products
</a>
<div class="modal hide" id='modal'>
  <div class='modal-body'>
    loading content..
   </div>
</div>

I press the link, it prompt the modal with one modal-backdrop, i close and reopen it, will displays two modal-backdrop, reopen N times, displays N modal-backdrop, like: