如何在不删除空行的情况下删除重复字符串？C#

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
        string input = "aabbccddeeff\n\naabbccddeeff\n\n";
        string result = RemoveDuplicateCharacters(input);
        Console.WriteLine(result);
    }

    static string RemoveDuplicateCharacters(string input)
    {
        var result = new List<string>();
        var lines = input.Split(new[] { '\n' }, StringSplitOptions.None);

        foreach (var line in lines)
        {
            var chars = line.ToCharArray();
            var uniqueChars = new HashSet<char>(chars);
            result.Add(new string(uniqueChars.ToArray()));
        }

        return string.Join("\n", result);
    }
}