If you get a line a bit like:
New client connection refused: Duplicate ID (a33f62d8).
You probably have copied your doomsday installation to/from other computers on your local network or copied them to a friends computer for Internet gaming?
The problem is caused by the client_id_number being stored in the file Run/jDoom/Client.ID (in versions prior to 1.9) and in Snowberry/Runtime/Client.ID (In version 1.9 and above.) Delete it (on the clients) and the error will disappear as Doomsday automatically recreates the file on startup.
version_1_9.0-beta5 supports the command line option -id for specifying the client ID on the command line.
The server requires a unique ID number for each client. The ID number is stored in the Client.ID file. If you copy a Doomsday installation from one computer to another, the ID number file is copied as well. This means when the second installation tries to connect to the first one, a duplicate ID error is reported and the connection is aborted. To fix this, you'll need to delete the Client.ID file from the copied installation. A new ID will be automatically generated when you start the game.