C#: Find the duplicates

//function to display the duplicate values in an Array
public void DisplayArray(ArrayList ary)
        {
//loop through all the elements
            for (int i = 0; i < ary.Count; i++)
            {
                Console.Write(ary[i]+" ");
            }
            Console.WriteLine();
        }
//function to find the duplicate values in an Array
  public void FindDuplicate(ArrayList ary)
        {
//Array list to store all the duplicate values
           ArrayList dup = new ArrayList();
          
            for (int i = 0; i < ary.Count;i++)
            {
                for (int j =i+1; j < ary.Count; j++)
                {
//compare each value with following remaining values
                    if (ary[i].Equals(ary[j]))
                    {
//When duplicate value is found, check
//whether the value not contained in the dup array list
                        if(!dup.Contains(ary[i]))
                        {
//if not contains, then add the value to dup array list
                       dup.Add(ary[i]);
                        }
                    }
                }
            }
            Console.WriteLine("The numbers which duplicates are");
            DisplayArray(dup);
        }


//Input Arraylist values: 4,5,2,5,4,7
 Output: 4 5 2 5 4 7
               The number which duplicates are
                4 5