Dedicated Server Kicking Roommate

[tcp]

He's mentioned that they have seperate copies.

It's probably not the configuration of the game, but the configuration of the router causing problems. What brand and model router are you using? Custom firmware?

[Torval]

And how exactly you have 2 different external IPs with 1 modem?

try set this kickduplicate=1; to this kickduplicate=0;

^^^ This worked for me. Same router, with a dedicated and 2 clients...all on separate PCs.