How to prevent double attributes

Hi,
I use FCK Editor Version 2.6.5 and java/jsp.
I try to demonstrate my problem by an example:

<ul id="1111">
<li id="xy1">some Text1</li>
<li id="xy2">some Text2</li>
</ul>

When the focus is in text1 and I press Enter :

<ul id="1111">
<li id="xy1">some text1</li>
<li id="xy1"></li>
<li id="xy2">some text2</li>
</ul>

Another li Element with the same attribute and the same value is created.
My question is: how to tell fck editor not to double the attribute and the attribute value?
Any hints are wellcome.

halmarius