How to append just one time

How to append just one time

Hi every, I have this code:

$(function(){
      $(".dialog_open").click(function(){
            $(this).append('<input type="text" name="new_qty" id="new_qty" value="" width="5" />');
       });
});

And this HTML markup:
<a class="dialog_open" onclick="javascript: return false" href="#">Rebajar</a>

When I click "Rebajar" the INPUT appears but any time I click in the INPUT one more appears and then another and another, why? What's wrong here?