var tempList = new string[]{ "1","2", "3", "4", "5", "4", "6" };

 for (int i = 0; i < tempList.Length; i++)
 {
     for (int j = i + 1; j < tempList.Length; j++)
     {
         if (tempList[i].Equals(tempList[j]))
         {
             MessageBox.Show("检测到有重复的数字 = " + tempList[i]);
             return;
         }
     }
 }