Code

[EditorBrowsable(EditorBrowsableState.Never)]
public void RemoveDuplicateItems()
{
	zOrder.RemoveDuplicates();
	nodes.RemoveDuplicates();
	links.RemoveDuplicates();
	groups.RemoveDuplicates();
} 

Code

diagram.ClearAll();

List<ShapeNode> nodes = new List<ShapeNode>();
for (int i = 0; i < 100; i++)
	nodes.Add(diagram.Factory.CreateShapeNode(i, i, 10, 10));

List<DiagramLink> links = new List<DiagramLink>();
for (int i = 0; i < 10; i++)
	links.Add(diagram.Factory.CreateDiagramLink(nodes[i], nodes[i + 1]));

// Add duplicates at random indices
diagram.ValidityChecks = false;
Random rnd = new Random();
for (int i = 0; i < 10; i++)
{
	int nodeIdx = rnd.Next(nodes.Count);
	int insertIdx = rnd.Next(diagram.Nodes.Count);
	diagram.Nodes.Insert(insertIdx, nodes[nodeIdx]);
}
for (int i = 0; i < 5; i++)
{
	int linkIdx = rnd.Next(links.Count);
	int insertIdx = rnd.Next(diagram.Links.Count);
	diagram.Links.Insert(insertIdx, links[linkIdx]);
}

int nodeCountWithDuplicates = diagram.Nodes.Count;
int linkCountWithDuplicates = diagram.Links.Count;

diagram.RemoveDuplicateItems();

int nodeCountAfterRepair = diagram.Nodes.Count;
int linkCountAfterRepair = diagram.Links.Count;

// This would crash without RemoveDuplicateItems call above
diagram.SaveToJson("test.json");
diagram.LoadFromJson("test.json");