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

 Hashtable ht = (Hashtable)Session["ht"]; i am using hashtable for storing values which i need to append to some query i have a function which has  Hashtable ht = (Hashtable)Session["ht"];   ht.Remove(HName.Value); this function gets executed on a postback click event of a button control as well as on another button control  which is needed as per my flow and i get teh error but without any postback if i go on stroing values  then i dont get how do i overcome this used is it possible to replace the existing key??