To broadcast to ALL connections including the caller you would use
Clients.All.showAlertMessage(message);
To broadcast to ALL EXCEPT the caller you will want to use the method
Clients.Others.showAlertMessage(message);
Can find more documentation on this at http://www.asp.net/signalr/overview/signalr-20/hubs-api/hubs-api-guide-server#callfromhub
This method above will broadcast to all connections except the current one which I believe is what you are looking for