duplicate the client's requests to another server

[Jeff Pang]

Hello,

When nginx pass the client's request to an original server, I want it
always duplicate the request to another webserver.

For example, the request is following: client -> nginx -> original-server
What I want is:

client -> nginx -> original-server
                     |-> another webserver

The request should always be duplicated, rather than the round robin.

How can I do that?

Thanks.