upstream name should be unique per entire config, you could name it in nginx.conf and than reference to its name from different your includes.
like:
upstream loadbalancer {
server 192.168.0.1:8080;
server 192.168.0.2:8080;
server 192.168.0.3:8080;
server 192.168.0.4:8080;
}

than in certain location (in included file for example)
proxy_pass http://loadbalancer;