To reproduce:

1. Drop RadApplicationMenu from the toolbox on to the windows form.
2. Click on the control to open menu and select "Add new > New RadMenuSeparatorItem" option to add new item.
3. Save the form.
4. Press Ctrl+Z to undo changes. Here added RadMenuSeparatorItem is removed from the control.
5. Now press Ctrl+Y to Redo the changes.
Here added item is not added again after performing Redo action.