i have some code like this

    $(".commentsubmit").click(function() {
     if ($(this).prev()[0].value != "") {
      var thiscomment = $(this).parents(".quick_comment_form");
      var cbox = thiscomment.next().next();
      var mbox = thiscomment.find(".commentmessage");
      var tbox = thiscomment.next();
      cbox.html("<img src=\"http://url\" /> Submitting");
      $.post("http://url",
      {message:mbox[0].value,target_id:tbox[0].value},
        function(data){
       cbox.html("");
       cbox.after(data);
       mbox.attr("value","");
       },"html"
      );
      return false;
     }
    });

the problem is that the data sometimes is submitted more than one time.

what to do about this

This depends on what element-type your submit is bound to.

You should use "event.preventDefault();" within your event handler to cancel any execution of the actual link element.

this is the code

  <div>
  <a href="#" class="quick_comment">+'.$flag.'</a> | '.$likeit.'
   <div class="quick_comment_form" style="display:none;">
    <table border="0"><tr><td valign="top">
    <img src="'xxxxxxxxx'" width="25" height="25" />
    </td><td>
    <textarea name="message" rows="2" class="fpost commentmessage""  style="width:350px;" ></textarea>
    <input type="button" value=" '.$flag.' " class="fbutton commentsubmit" />
    </td></tr>
    </table>
   </div>
   <input type="hidden" name="wall_id" value="xx'" />
   <div></div>
  </div>';

You are binding your event handler to both the anchor and the button. in case of the button, you are not preventing the default behaviour of the button.

Change it like that:

    $(".commentsubmit").click(function(event) {
     if (!event) { event = window.event;}
     if (event.preventDefault) { event.preventDefault();}
     if (event.stopPropagation) { event.stopPropagation();}

     if ($(this).prev()[0].value != "") {