Cannot join a multiplayer TFC game when friend on the same network is on the server

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When I try playing the game Team Fortress Classic I can join and play fine but if my friend on the same network as mine is already in the game or im already in it we both lock up and cant join the game, getting disconnected. Thing is, it works on other multiplayer games like Americas Army and Age of Empires but when we play TFC thats when we have problems. We cant join the same game. I tried changing the domain and workgroup to try and distinguish our computers a little better but it no worky. Any ideas?