Session Variable Gets Changed

Question

Hi All,
       Iam Using Session to pass values to another Page. Iam using ArrayList in Session.
In next page am deleting some list items 

In Session["ContrcatList"] i have around 20 items and i delete some items based on contractId which is unique for each item in array list, but when i again use Session["ContractList"] this get modified(ArrayList after deletion of some items) and i dont get the orginal value. How i will get original value and i need 2 modify Array list as well.

 ArrayList listOfContract = null;               

  listOfContract = (ArrayList)Session["ContractList"]; // here listOfContract.Count = 20

ArrayList ar1 = new ArrayList();

ar1 = listOfContract;

 ar1[0] = listOfContract[contractId];

 ar1.RemoveRange(1, ar1.Count - 1);

 ArrayList listOfContract1 = null;               

  listOfContract1 = (ArrayList)Session["ContractList"]; // here listOfContract.Count = 1;