// Add
public TreeViewLayer TreeView;

public void AddLayer(Layer layer)
{
    var node = new TreeNode<Layer>(layer);
    TreeView.Nodes.Add(node);
}

// Duplicate
public void Duplicate(TreeNode<Layer> node)
{
    var layer = node.Item; // maybe it also should be copied
    var duplicated_node = new TreeNode<Layer>(layer, CreateNodesCopy(node.Nodes)); // create the new node with duplicated nested nodes
    duplicated_node.Parent = node.Parent; // add node to the same branch
}

ObservableList<TreeNode<Layer>> DuplicateNodes(ObservableList<TreeNode<Layer>> nodes)
{
    var result = new ObservableList<TreeNode<Layer>>();
    if (nodes == null)
    {
        return result;
    }

    foreach (var node in nodes)
    {
        result.Add(Duplicate(node));
    }

    return result;
}

// Remove
public void Remove(TreeNode<Layer> node)
{
    node.Parent = null;
}

// Serialization
public List<TreeNodeSerialized<Layer>> Serialize()
{
    return TreeNode<Layer>.Serialize(TreeView.Nodes); // convert nodes to simple list than can be easily serialized
}

public void Deserialize(List<TreeNodeSerialized<Layer>> nodes)
{
    TreeView.Nodes = TreeNode<Layer>.Deserialize(nodes);
}