Add only new items from listbox to SQL

i have a listbox that users can add Countries Names into it anytime. now i need to only add new data to check if the data is new then insert it to sql. my code not work:

For Each i As String In listbox1.Items
    Dim sql = "select * From Countries where CountryName=N'" & i & "'"
    Dim adp As New SqlClient.SqlDataAdapter(sql, SQlconn)
    Dim ds As New DataSet
    adp.Fill(ds)
    Dim dt = ds.Tables(0)
    If dt.Rows.Count = 0 Then
        Dim dr = dt.NewRow
        dr!CountryName = i
        dt.Rows.Add(dr)
        Dim cmd As New SqlClient.SqlCommandBuilder(adp)
        adp.Update(dt)
    End If
Next

If you're loading the existing names from the database in the first place then there's no need for you to check anything. Simply query the database to populate a DataTable and bind that to the ListBox. When the user adds a new country, add it to the DataTable and it will automatically show up in the ListBox. When it's time to save, just use the same data adapter to save the DataTable and only the new records will be saved.