Run the Message Server (Samba.MessagingServer.exe) ONLY ON ONE computer. Easiest way is to put a short cut in your Start Menu Startup folder, and make sure its set to Start Server Automatically.
Then on every Terminal (including the terminal running Message Server) go to Settings->Local Settings, and set Message Server Name as the computer name of the terminal running the Message Server, Tick Auto Start Message Client, save, and restart SambaPOS.