By default, we can’t add the same instance of Node to different diagram control. Currently, we don’t have an option to achieve this requirement. This is not an issue. This is default framework behaviour. Because, we can’t add the same instance of Content Control as child / content of two or more panel / UIElement.
So, we are suggesting you to please remove the Node from the dummy diagram control before adding the Node to main diagram control.
Please let us know if you need further assistance on this.
Sorry for inconvenience.
As the reported query was the default framework behaviour, we were unable to suggest you with a better solution.
Please let us know if you have any other concern.