using System.Collections;
using System.Linq;

ArrayList duplicateList= new ArrayList();
duplicateList.Add(10);
duplicateList.Add(20);
duplicateList.Add(10); 
duplicateList.Add(30);
duplicateList.Add(40); 
duplicateList.Add(50);
duplicateList.Add(40); 
duplicateList.Add(20);

public ArrayList RemoveDuplicate(ArrayList duplicate)
{
       ArrayList distinctArr= new ArrayList();       duplicate.ToArray().Distinct().ToList()
                              .ForEach(d=> distinctArr.Add();
       return distinctArr;
}


 ArrayList distinctList= new ArrayList();
 distinctList = RemoveDuplicate(duplicateList);

using System.Collections;
protected void Page_Load(object sender, EventArgs e)
        {
ArrayList arrLs = new ArrayList { 1, 4, 15, 3, 7,4,1,3 };
ArrayList distinctArrLs = RemoveDuplicate(arrLs);
}


    private static ArrayList RemoveDuplicate(ArrayList sourceList)
    {
        ArrayList list = new ArrayList();
        foreach (int item in sourceList)
        {
            if (!list.Contains(item))
            {
                list.Add(item);
            }
        }
        return list;
    }