WPF vb.net ShowModelessWindow prevent multiple instances

[budinsky]

I would like to have a MainWindow open as modeless so it can stay open, but calling the command again results in multiple modeless windows. How can I check if the MainWindow is already open to prevent multiples?

<CommandMethod("BatchProcess")> Public Sub cmdBatchProcess()
  Dim winBatPro As New MainWindow
  Application.ShowModelessWindow(winBatPro)
End Sub