using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            List<char> c = GetDuplicates("Hiii").ToList();
            for (int i = 0; i < c.Count; i++)
            {
                Console.WriteLine(c[i]);
            }
            Console.Read();
        }

        public static List<char> GetDuplicates(string text)
        {
            List<char> duplicates = new List<char>();
            char[] chars = text.ToLower().ToCharArray();

            for (int i = 0; i < chars.Length; i++)
            {
                char current = chars[i];

                if (i + 1 >= chars.Length)
                    if (duplicates.Contains(current))
                    {
                        duplicates.Add(current);
                        break;
                    }
                for (int j = i + 1; j < chars.Length; j++)
                {
                    if (current == chars[j] || duplicates.Contains(current))
                    {
                        duplicates.Add(current);
                        break;
                    }
                }
            }

            return duplicates;
        }
    }
}