using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var vs = new List<Vertex>

        {

            new Vertex { X = 1, Z = 1, Y = 3 }, // Group 1

            new Vertex { X = 1, Z = 1, Y = 2 }, // Group 1

            new Vertex { X = 1, Z = 1, Y = 6 }, // Group 1

            new Vertex { X = 1, Z = 2, Y = 1 }, // Group   2

            new Vertex { X = 2, Z = 1, Y = 1 }, // Group     3

            new Vertex { X = 1, Z = 2, Y = 3 }  // Group   2

        };

        var vGroups = vs.AsParallel().GroupBy(vertex => new { vertex.X, vertex.Z });

        foreach (var vGroup in vGroups)

        {

            vGroup.Aggregate((prev, curr) =>

                {

                    // If prev is null (i.e. first iteration of the "loop") don't change the 'Y' value

                    curr.Y = prev?.Y ?? curr.Y;

                    return curr;

                });

        }

        FiddleHelper.WriteTable(vs);

    }

}

​

public class Vertex

{

    public int X { get; set; }

    public int Y { get; set; }

    public int Z { get; set; }

}