A question about add_header replication

[bit bull]

hi,all:

    hello, I config "add_header Server xxxx;", then I get response header:

< HTTP/1.1 200 OK
< Server: nginx/0.8.17
< Date: Mon, 05 Oct 2009 07:59:18 GMT
< Content-Type: text/html
< Content-Length: 151
< Last-Modified: Mon, 05 Oct 2009 07:24:51 GMT
< Connection: keep-alive
< Server: xxxx
< Accept-Ranges: bytes

We can see Server field is printed twice. I need replace first one.

So I read source code, found:

1.add_header is simple push to header_out array, keep
headers_out.server == NULL.
2. "Server:nginx/0.8.17" field is setted at http_header_filter_module.c:457
3. a loop start at http_header_filter_module.c:568, it concat all
header fields(include "Server xxxx").
4. Server field is replication.

why not set headers_out.server at add_header? how to implement my need?

sorry for my poor english.

thanks
bb