using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var data = new List<List<string>>() 

        { 

            new List<string>() { "1", "4", "6", "8" }, 

            new List<string>() { "1", "2", "6", "8" }, 

            new List<string>() { "2", "4", "6", "8" }, 

            new List<string>() { "3", "4", "5", "7" }, 

        };

        foreach(var row in data)

        {

            foreach(var column in row)

            {

                Console.Write(column);

            }

            Console.WriteLine();

        }

        var result = FilterData(data);

        Console.WriteLine("----------------");

​

        foreach(var row in result)

        {

            foreach(var column in row)

            {

                Console.Write(column);

            }

            Console.WriteLine();

        }

    }

    private static List<List<string>> FilterData(List<List<string>> datatable)

    {

        var result = new List<List<string>>();

        for(var rowindex = 0; rowindex < datatable.Count; rowindex++)

        {

            var refrow = datatable[rowindex].Select(item => (string)item.Clone()).ToList();

            result.Add(refrow);

            if (rowindex == 0) continue;

​

            var row = result[rowindex];

            var prevrow = datatable[rowindex - 1];

            for(var columnindex = 0; columnindex < row.Count; columnindex++)

            {

                if (row[columnindex] == prevrow[columnindex])

                    row[columnindex] = "-";

            }

        }

        return result;

    }

}