problem with a dropDownList => in inside of Page_Load method....

i have this code (when autoPostBack is set to true for the dropDownList)
DropDownList1.Items.Insert(0, "None")
DropDownList1.Items.Insert(1, "ID")
DropDownList1.Items.Insert(2, "UserName")
DropDownList1.Items.Insert(3, "FirstName")
DropDownList1.Items.Insert(4, "LastName")
DropDownList1.DataBind()
the problem is when i'm clicking and changing the selectedIndex at the dropDownList. the dropDownList creates new 4 elements everytime. how can i fix this problem.
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