LAN mode in Mario Kart Double Dash (GameCube), can't establish connection

[shuffle2]

Did you copy your dolphin configuration from one PC to another? Your problem may be that they are using the same MAC address.
You can delete the MAC setting in the dolphin configuration file and dolphin will generate a new one.

Also, make sure your router supports UPnP and it's enabled. The UI of your router probably has a place to view active UPnP endpoints, as well.