7(my)

using System;

namespace ConsoleApp5
{
    class Program
    {
        static void Main()
        {
            var str = "asxd xsa";
            var result = "";
            for (int i = 0; i < str.Length; i++)
            {
                bool end = true;
                for (int j = i + 1; j < str.Length; j++)
                {
                    if (str[i] == str[j])
                    {
                        result += str[i];
                        end = false;
                        break;
                    }
                }
                if (end == true)
                    break;
            }
            Console.WriteLine(result);
            Console.ReadKey();
        }
    }
}