I have service InstanceContextMode sets to Single mode.
I'm connecting with many clients to this service, getting it's callback channel with OperationContext.Current.GetCallbackChannel<IMyService>() and tring to send some message to each connected client, but I get the same message twice in only one client (this one which was conneceted later)
If I have InstanceContexMode PerSession everything works great.
What did I missed? Because be default I suppose it is possible, right?
WSDualHttpBinding (aka "duplex" binding) requires session to work correctly. The session is used to keep track of the callback channel on a per-client basis, IIRC.