Add/Remove unorder lists/sublists (two-level) Jquery - nested-lists

Question

I want to create a form using an unordered list ul must have only two levels which I am not getting in this demo:
http://jsfiddle.net/rathoreahsan/9SCCn/
When I add a sub-list it adds one sub list of parent list (it is fine) but when I add another sub-list it adds a sub-list for newly added sub-list (that i don't want). I want when I add sub-list it only add 1 sub-list of a parent list instead of adding sub-list of newly added sub-lists.
Current Structure:
<ul>
<li><input type="text" />
<ul>
<li><input type="text" /></li> // New added sub list
<ul> //It should'nt be like this when added more
<li><input type="text" /></li>
</ul>
</li>
</ul>
</li>
</ul>
The structure should be like this:
<ul>
<li><input type="text" />
<ul>
<li><input type="text" /></li> // New added sub list
<li><input type="text" /></li> //It should be like this when added more
</ul>
</li>
</ul>
Edited
Finally got the solution See Workin Demo: http://jsfiddle.net/rathoreahsan/2hwLp/

Answer 1

JQUERY
$.fn.exists = function(){ return this.length>0; }
function dBtnClick(val){
if( $('#duties li#' + val + ' ul.sublists').exists() )
{
$('#duties li#' + val + ' ul.sublists').append('<li><input type="text" /></li>')
}
else if( !$('#duties li#' + val + ' ul.sublists').exists() )
{
$("#duties li#" + val).append('<ul class="sublists"><li><input type="text" /></li></ul>')
}
}
var dutiesListID = $('#duties li#0').attr('id');
var d_cur_no = parseInt(dutiesListID);
$('#dr').click(function(){
d_cur_no++;
$('#duties').append('<li id="' + d_cur_no + '"><input type="text" /> Add Sub List Item</li>');
});
HTML:
<h3>Duties and Responsibilities:</h3>
<ul id="duties">
<li id="0"><input type="text" /> Add Sub List Item</li>
</ul>
<span class="controls">
Add List Item
</span>
DEMO:
http://jsfiddle.net/rathoreahsan/2hwLp/

Answer 2

This might work:
change html part here:
<ul id="duties">
<li class="top-level"><input type="text" /></li>
</ul>
and js part here:
$('#dr-sub').click(function(){
$('.top-level').last().append('<ul><li><input type="text" /></li></ul>');
});

Answer 3

Try to change it like this:
$('#duties li').last().append('<li><input type="text" /></li>');