Problem with connection in WCF application

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WCF

I migrate our application into a WCF application. I've added a login screen and I create the channel for each user Calback:

client:

_ChannelFactory = New DuplexChannelFactory(Of IService)(interface_de_rappel,New EndpointAddres, New EndpointAddress(Config.AdresseServeur))
 _Clt = CType(_ChannelFactory.CreateChannel, OCServeur.IService)

server:

_Callback = OperationContext.Current.GetCallbackChannel(Of IRappel)()

when users do not connect at the same time, all is well! but if they connect at the same time t (a roughly), one or two users come n not connect and it displays the following bug:

The object of communication System.ServiceModel.Channels.ServiceChannel can not be used for communication because it has been canceled.

Server stack trace: 
  à System.ServiceModel.Channels.CommunicationObject.ThrowIfDisposedOrNotOpen()
  à System.ServiceModel.Channels.ServiceChannel.EnsureOpened(TimeSpan timeout)
  à System.ServiceModel.Channels.ServiceChannel.Call(String action, Boolean oneway, ProxyOperationRuntime operation, Object[] ins, Object[] outs, TimeSpan timeout)
  à System.ServiceModel.Channels.ServiceChannel.Call(String action, Boolean oneway, ProxyOperationRuntime operation, Object[] ins, Object[] outs)
  à System.ServiceModel.Channels.ServiceChannelProxy.InvokeService(IMethodCallMessage methodCall, ProxyOperationRuntime operation)
  à System.ServiceModel.Channels.ServiceChannelProxy.Invoke(IMessage message)

[edit]Subject changed to reflect language now English - OriginalGriff[/edit]