I realize that these questions are similar:
SignalR - Broadcast to all clients except Caller
Send a message to all clients in a Group, except for the current cl
If you're sending the message in a method on the Hub class, you can also use Clients.Others to exclude the calling client. You only need to use AllExcept(id) if you're not in a hub method.
See the following for details:
http://www.asp.net/signalr/overview/hubs-api/hubs-api-guide-server#selectingclients