using System.Collections.Generic;

using System.Linq;

using System;

​

public class Program

{

    public static void Main()

    {

        var items = new List<List<string>>{

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

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

            new List<string>{ "9", "4", "6", "8" },

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

        };

​

        var result = items.RemoveDuplicates("-");

        result.ForEach(m=>{

            Console.WriteLine(string.Join(",", m));

        }); 

    }

}

​

​

    public static class Extensions

    {

        public static List<List<T>> RemoveDuplicates<T>(this List<List<T>> items, T replacedValue) where T: class

        {

            var ret = new List<List<T>>();

            items.ForEach(m=> {

                var ind = items.IndexOf(m);

                if(ind==0)

                {

                    ret.Add(items.FirstOrDefault());

                }

                else

                {

                    var prevItem = items.Skip(items.IndexOf(m)-1).FirstOrDefault();

                    var item = new List<T>();

                    for(var a = 0; a < prevItem.Count; a++)

                    {

                    item.Add(prevItem[a] == m[a]? replacedValue : m[a]);

                    }

                    ret.Add(item);

                }

            });

            return ret;

        }

    }