using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var myList = new List<int> { 1, 1, 9, 2, 2, 3, 9, 4, 5, 5, 1, 9, 9 };

        var firstDuplicate = myList.First();

        var lastDuplicate = myList.Last();

​

        var myFilteredList = myList

            .SkipWhile(l => l == firstDuplicate)

            .SkipLastWhile(l => l == lastDuplicate)

            .ToList();

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

    }

}

​

public static class Helper

{

    public static IEnumerable<TSource> SkipLastWhile<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)

    {

        var buffer = new List<TSource>();

​

        foreach (var item in source)

        {

            if (predicate(item))

            {

                buffer.Add(item);

            }

            else

            {

                if (buffer.Count > 0)

                {

                    foreach (var bufferedItem in buffer)

                    {

                        yield return bufferedItem;

                    }

​

                    buffer.Clear();

                }

​

                yield return item;

            }

        }

    }

}