Как получить 1 значение для каждой провинции в combobox

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когда я выбираю регион, он показывает следующий регион, а когда я иду в провинцию, он показывает много провинций повторно, как, например, Себу, он показывает все Себу в combobox
все, что я хочу, это только 1 Себу будет видеть в combobox вместо многих Себу. то же самое с городом

Что я уже пробовал:

Private Sub cboRegion_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cboRegion.SelectedIndexChanged
        Dim oledr As OleDb.OleDbDataReader
        Connection = "select * from Address where Region = '" & cboRegion.Text & "'"
        cboProvince.Items.Clear()
        Dim acscmd As New OleDb.OleDbCommand
        acscmd.CommandText = Connection
        acscmd.Connection = OleCn
        oledr = acscmd.ExecuteReader()

        While (oledr.Read())
            cboProvince.Items.Add(oledr("Province"))
        End While
        cboProvince.SelectedIndex = -1
        acscmd.Dispose()
        oledr.Close()
    End Sub

Private Sub cboProvince_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cboProvince.SelectedIndexChanged
        Dim oledr As OleDb.OleDbDataReader
        Connection = "select * from Address where Province = '" & cboProvince.Text & "'"
        cboCity.Items.Clear()
        Dim acscmd As New OleDb.OleDbCommand
        acscmd.CommandText = Connection
        acscmd.Connection = OleCn
        oledr = acscmd.ExecuteReader()

        While (oledr.Read())

            cboCity.Items.Add(oledr("City"))

        End While
        cboCity.SelectedIndex = -1

        acscmd.Dispose()
        oledr.Close()
    End Sub

Private Sub cboCity_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cboCity.SelectedIndexChanged
       Dim oledr As OleDb.OleDbDataReader
       Connection = "select * from Address where City = '" & cboCity.Text & "'"
       cboBrgy.Items.Clear()
       Dim acscmd As New OleDb.OleDbCommand
       acscmd.CommandText = Connection
       acscmd.Connection = OleCn
       oledr = acscmd.ExecuteReader()
       While (oledr.Read())
           cboBrgy.Items.Add(oledr("Barangay"))
       End While
       cboBrgy.SelectedIndex = -1
       acscmd.Dispose()
       oledr.Close()

   End Sub

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