Web Forms :: Stop Inserting Duplicate Value In Ddl Using Code Behind

my ddl1 and ddl2 in update panel so i want to insert item in ddl using following condition. It is working fine but when you choose ddl2 more than one then it insert item every time meand duplicate value and populate ddl1 every time.

protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
{
if (DropDownList2.SelectedValue=="1" )
{
DropDownList1.Items.Insert(0, new ListItem("first", "1"));
DropDownList1.Items.Insert(1, new ListItem("Second", "2"));
DropDownList1.Items.Insert(2, new ListItem("Third", "3"));
}
else if (DropDownList2.SelectedValue == "2" )
{
DropDownList1.Items.Insert(0, new ListItem("first", "1"));
DropDownList1.Items.Insert(1, new ListItem("Second", "2"));
}
}