ZohoCRM can't insert Tasks with the same Subject

I have a list of "external" (to ZohoCRM) Tasks I want to import to ZohoCRM Tasks.

There are tasks which contain the same Subject as well as other fields (but sometimes they are different). For example:

<Tasks>

<row no="1">

<FL val="Task Owner"><![CDATA[Jolanta]]></FL>

<FL val="Subject"><![CDATA[EaV 100]]></FL>

<FL val="Due Date"><![CDATA[2030-08-20]]></FL>

<FL val="Status"><![CDATA[Anulowane]]></FL>

<FL val="Description"/>

</row>

</Tasks>

 <Tasks> 
<row no="1"> 
<FL val="Task Owner"><![CDATA[Jolanta]]></FL> 
<FL val="Subject"><![CDATA[EaV 100]]></FL> 
<FL val="Due Date"><![CDATA[2030-08-15]]></FL> 
<FL val="Status"><![CDATA[Anulowane]]></FL> 
<FL val="Description"/> 
</row> 
</Tasks>
Requirement is that I need to import them both as two tasks. But Zoho gives me

this response when trying to `insertRecords` with second xml chunk:

 <?xml version="1.0" encoding="UTF-8"?> 
<response uri="/crm/private/xml/Tasks/insertRecords"> 
<result> 
<message>Record(s) already exists</message> 
<recorddetail> 
<FL val="Id">656609000000159069</FL> 
<FL val="Created Time">2012-09-06 16:32:23</FL> 
<FL val="Modified Time">2012-09-06 16:32:23</FL> 
<FL val="Created By"><![CDATA[AAA]]></FL> 
<FL val="Modified By"><![CDATA[AAA]]></FL> 
</recorddetail> 
</result> 
</response>
How can I add tasks with the same Subject and for example

different `Description`/`<custom field>`?

(I can do it using web interface but cannot using API)