ArgumentException: An item with the same key has already been added

[andyste1]

Using v3.2.1 to support dragging and dropping of items in a ListBox. Basic stuff really, with these attached properties on the ListBox:

dd:DragDrop.IsDragSource="True" dd:DragDrop.IsDropTarget="True" dd:DragDrop.DropHandler="{Binding}"

And the VM implementing IDropTarget.

It's worked fine for a couple of years, but one user recently encountered an error when dragging/dropping. It has never been reported elsewhere and I haven't been able to reproduce it, so I'd like to chalk it up to a "one off" glitch. However I thought I'd better ask here, just in case anyone can shed light on what might have caused it. This is the stack trace:

System.ArgumentException: An item with the same key has already been added. Key: System.Windows.Controls.ItemsControl+ItemInfo
at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
at System.Windows.Controls.Primitives.Selector.SelectionChanger.ApplyCanSelectMultiple()
at System.Windows.Controls.Primitives.Selector.SelectionChanger.End()
at System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
at System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType)
at System.Windows.DependencyObject.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType, Boolean isInternal)
at System.Windows.DependencyObject.SetCurrentValue(DependencyProperty dp, Object value)
at GongSolutions.Wpf.DragDrop.Utilities.ItemsControlExtensions.SetSelectedItem(ItemsControl itemsControl, Object item2Select) in C:\projects\gong-wpf-dragdrop\src\GongSolutions.WPF.DragDrop\Utilities\ItemsControlExtensions.cs:line 349
at GongSolutions.Wpf.DragDrop.DragDrop.DragSourceUp(Object sender, Point elementPosition) in C:\projects\gong-wpf-dragdrop\src\GongSolutions.WPF.DragDrop\DragDrop.cs:line 551
at GongSolutions.Wpf.DragDrop.DragDrop.DragSourceOnMouseLeftButtonUp(Object sender, MouseButtonEventArgs e) in C:\projects\gong-wpf-dragdrop\src\GongSolutions.WPF.DragDrop\DragDrop.cs:line 499
at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.ReRaiseEventAs(DependencyObject sender, RoutedEventArgs args, RoutedEvent newEvent)
at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
at System.Windows.UIElement.RaiseTrustedEvent(RoutedEventArgs args)
at System.Windows.Input.InputManager.ProcessStagingArea()
at System.Windows.Input.InputProviderSite.ReportInput(InputReport inputReport)
at System.Windows.Interop.HwndMouseInputProvider.ReportInput(IntPtr hwnd, InputMode mode, Int32 timestamp, RawMouseActions actions, Int32 x, Int32 y, Int32 wheel)
at System.Windows.Interop.HwndMouseInputProvider.FilterMessage(IntPtr hwnd, WindowMessage msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at System.Windows.Interop.HwndSource.InputFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)