update solr.xml error after create a new core

[kelong]

Hi
    I create a new core dynamically with

http://localhost:8983/solr/admin/cores?action=CREATE&name=mycore&collection=collection1&shard=shard2&instanceDir=.&dataDir=example2\solr\data

    Before creating the solr.xml is

* <solr persistent="true">*
*    <core name="collection1" instanceDir="." shard="shard1"/>*
*    </cores>*
* </solr>*

     After creating, the solr.xml becomes

<solr persistent="true">
  <cores defaultCoreName="collection1" adminPath="/admin/cores"
zkClientTimeout="10000" hostPort="8983" hostContext="solr">
    <core shard="shard2" instanceDir=".\" name="mycore"
dataDir="\example2\solr\data" collection="collection1"/>
    <core shard="shard2" instanceDir=".\" name="mycore"
dataDir="\example2\solr\data" collection="collection1"/>
  </cores>
</solr>

the new core is added twice and the old one information is missed. Can any
one exmplain why this happens? Is there any error in my operation?

Thanks!