Item has already been added. Key in dictionary: CITY Key being adde

The above error occurs when you try to insert a key that already exists and since sessions are available in postback and also partial postback your old values are retained in session hence I would suggest check whether key exists and if it does not then insert

ya u r ight the key exists

so i get the error , but how do i update or replace it

any sample example

and whts the major difference between normal array & hashtable in terms of working & speed & any other property in dotnet

if (ht.ContainsKey(HName.Value))
        {
            ht[HName.Value] = "new value";
        }