string str1="1,2,3,4,5,6,7,1,2,3,4,5";

string str3="1,2.3,4,5,6,7";

 string[] stest = new string[] { "aaa", "bbb", "ccc", "aaa", "ccc" };

            ArrayList list = new ArrayList();

            for (int i = 0; i < stest.Length; i++)
            {
                bool IsExist = true;

                for (int j = 0; j < list.Count; j++)
                {

                    if (list[j].ToString() == stest[i])
                    {

                        IsExist = false;

                        break;

                    }

                }
                if (IsExist)
                {
                    list.Add(stest[i]);
                }
            }

------解决方案--------------------
C# code

string strA="a,b,c,d,e,f";
string strB="a,b,d,f,g,h";
string[] ArrayA=strA.split(',');
string[] ArrayB=strB.split(',');
string[] ArrayB= ArrayA.Union(ArrayB.Where(str => !ArrayA.Contains(str))).ToArray();

------解决方案--------------------
C# code

using System.Linq;


 string origin = "1,2,3,4,5,6,7,4,5,7";
var array = origin.Split(new string[]{","}, StringSplitOptions.RemoveEmptyEntries).Distinct().ToArray();
string des = String.Join(",", array);
Console.WriteLine(des);

------解决方案--------------------
            string str = "1,2,3,4,5,6,7,1,2,3,4,5";
           ArrayList arr = new ArrayList();
           arr = CleanRepeat(str);
           foreach (object var in arr )
           {
               Console.WriteLine(var.ToString());
           }
           Console.ReadLine();


static ArrayList CleanRepeat(string str)
       {
           string[] temp;
           ArrayList arraylist = new ArrayList();
           temp = str.Split(',');
           arraylist.Add(temp[0]);
           for (int i = 0; i < temp.Length; i++)
           {
               if (arraylist.IndexOf(temp[i]) != -1)
               {
                   continue;
               }
               else
               {
                   arraylist.Add(temp[i]);
               }
           }
           return arraylist;   
       }

------解决方案--------------------
C# code

            string str1 = "1,2,3,4,5,6,7,1,2,3,4,5";
            string str3 = null;
            for (int i = 0; i < str1.Length; )
            {
                string str2 = str1.Substring(i, 1);
                if (i == 0) str3 += str2;
                if (i >= 2 && str3.IndexOf(str2) < 0)
                    str3 += "," + str2;
                i = i + 2;
            }

------解决方案--------------------
string str1 = "1,2,3,4,5,6,7,1,2,3,4,5";
str1 = string.Join(",", str1.Split(',').Distinct().ToArray());