When I try it out, the definition that comes later seems to take effect. The question is, are there unintended consequences of doing this?

http {
map $http_host $a {
hostnames;
default 1;
example.com 1;
*.example.com 2;
}
map $http_host $a {
hostnames;
default 3;
example.com 3;
*.example.com 4;
}
server {
server_name example.com *.example.com
location / {
echo $a
}
}
}
Now with this configuration if I try:

curl http://example.com
3
curl http://www.example.com
4
Background: We are using this to provide an override for a map in an optional include file. So right after the map is defined, we have an include directive for *_override_map.conf. If this file exists and provides an alternate definition for the same map, then thats what gets used instead of the original map definition.