string s = "101,102,103,104,105,101,102,103,104,105,106,107,101,108";
s = string.Join(",", s.Split(',').Distinct().ToArray());

class Program
    {
static void Main(string[] args)
        {
string result="";
string str = "101,102,103,104,105,101,102,103,104,105,106,107,101,108";
            ArrayList list = array(str);
for (int i = 0; i < list.Count;i++)
            {
if (i == list.Count - 1)
                {
                    result += list[i];
                }
else
                {
                    result += list[i] + ",";
                }
            }
            Console.WriteLine(result);

            Console.ReadKey();
        }

static ArrayList array(string str)
        {
            ArrayList aimArr = new ArrayList();
            ArrayList strArr = new ArrayList();
string [] strs=str.Split(',');
foreach (string s in strs)
            {
                strArr.Add(s);
            }
for (int i = 0; i < strs.Length; i++)
            {
if (!aimArr.Contains(strs[i]))
                {
                    aimArr.Add(strs[i]);
                }
            }
return aimArr;
        }
    }

//101,102,103,104,105,106,107,108

            string input = "101,102,103,104,105,101,102,103,104,105,106,107,101,108";
            input = Regex.Replace(input + ",", @"(?:([^,]+,))(?=.*?\1)", "");
            Console.WriteLine(input.Substring(0,input.Length-1));