Failede to configure default document

Hi,

I'm trying to configure the default document in web.config.Everything seems to be correclty. However, I got the error message 

 Cannot add duplicate collection entry of type 'add' with unique key attribute 'value' set to 'default.aspx'.

My configuration is:

<defaultDocument> 

<files> 

<add value="default.aspx" /> 

</files> 

</defaultDocument>

 I'm not sure what's wrong with this section.

Any suggestions will be appreciated.