class Program
    {
        static void Main(string[] args)
        {
            string result = AmountSubstring("tebidohtebidoh");

        }

        static string AmountSubstring(string str1)
        {
            Go:
            string substring = "";
            int amountSubstringStr = 0;
            int maxAmount = 0;
            string maxSubstring = "";

            int g = 1;
            int i = 0;

            char[] arrayCharStr = new char[str1.Length];

            foreach (var el in str1)
            {
                arrayCharStr[i] = el;
                i++;
            }

            substring = substring + arrayCharStr[0];

            for (int l = 1; l < arrayCharStr.Length; l++)
            {
                substring += arrayCharStr[l];

                amountSubstringStr = new Regex(substring).Matches(str1).Count;

                if (amountSubstringStr >= maxAmount)
                {
                    maxAmount = amountSubstringStr;
                    maxSubstring = substring;
                }
            }

            if (maxAmount < 2)
            {
                str1 = str1.Substring(g);
                g++;
                goto Go;
            }
            return maxSubstring;
        }
    }