Remove Duplicates Extension

Remove Duplicates Extension From List C#

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;
    }
}

DEMO