compare lists

This problem is easier to solve if we first sort the ArrayList (or a copy thereof) so that any duplicated points are adjacent to each other. As the System.Drawing.Point struct doesn't implement IComparable, we need to create a custom IComparer class to do this.

Below, I've coded a quick console app to illustrate this approach with some sample data which contains duplicates:

using System;
using System.Collections;
using System.Drawing;

class Test
{
    static void Main()
    {
        ArrayList points = new ArrayList();
        points.Add(new Point(10,20));
        points.Add(new Point(20,40));
        points.Add(new Point(10,20));
        points.Add(new Point(30,50));
        points.Add(new Point(10,40));
        points.Add(new Point(10,20));
        points.Add(new Point(30,40));
        points.Add(new Point(30,50));

        // clone the ArrayList so that we can sort it without affecting original
        ArrayList points2 = (ArrayList)points.Clone();
        points2.Sort(new PointComparer());

        // now check for duplicates
        bool anyDuplicates = false;
        int count = 1;       

        for (int i = 1; i < points2.Count; i++)
        {
           if (points2[i].Equals(points2[i-1]))
           {
              count++;
              if (!anyDuplicates) anyDuplicates = true;
              if (i == points2.Count - 1)
              {
                Console.WriteLine("{0} occurred {1} times", points2[i], count);
              }
           }
           else
           {
              if (count > 1)
              {
                Console.WriteLine("{0} occurred {1} times", points2[i-1], count);
                count = 1;
              }
           }        
        }

        if (!anyDuplicates)
           Console.WriteLine("There are no duplicated points");
        Console.ReadLine();
    }

}

class PointComparer : IComparer
{
      public int Compare (object o1, object o2)
      {
         Point p1 = (Point)o1;
         Point p2 = (Point)o2;
         if (p1.X < p2.X || (p1.X == p2.X && p1.Y < p2.Y))
            return -1;
         else if (p1 == p2)
            return 0;
         else
            return 1;
      }
}