Code

public class MyBehavior : LinkShapesBehavior
{
	protected internal MyBehavior(DiagramView diagramView) : base(diagramView)
	{
		diagramView.ModifierKeyActions.Control = ModifierKeyAction.None;
	}

	public override InteractionState StartDraw(PointF point)
	{
		if ((Control.ModifierKeys & Keys.Control) != 0)
		{
			var node = Diagram.GetNodeAt(point);
			if (node != null)
			{
				node = (DiagramNode)node.Clone(false);
				Diagram.Nodes.Add(node);
				return new InteractionState(node, 8, Action.Modify);
			}
		}
		return base.StartDraw(point);
	}
} 

Code

public class MyBehavior : LinkShapesBehavior
{
	protected internal MyBehavior(DiagramView diagramView) : base(diagramView)
	{
		diagramView.ModifierKeyActions.Control = ModifierKeyAction.None;
	}

	public override InteractionState StartDraw(PointF point)
	{
		if ((Control.ModifierKeys & Keys.Control) != 0)
		{
			var node = Diagram.GetNodeAt(point);
			if (node != null)
			{
				var selectionCopy = Diagram.CopySelection(Diagram, false, true);
				Diagram.PasteSelection(Diagram, selectionCopy, null, 0, 0);
				return new InteractionState(Diagram.Selection, 8, Action.Modify);
			}

			return new InteractionState(Diagram.Selection, -1, Action.Create);
		}
		return base.StartDraw(point);
	}

	public override bool ShouldMoveSelection(PointF point)
	{
		return (Control.ModifierKeys & Keys.Control) == 0;
	}
}