How to avoid adding duplicate items in a ComboBox in VB ?

Dim conn9 As New OleDbConnection

            conn9.ConnectionString = ("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=d:\store1.mdb;Jet OLEDB:Database Password=123456")

            conn9.Open()

            Dim strsql9 As String

            strsql9 = "select * from types where code1='" + TextBox6.Text + "' "

            Dim cmd9 As New OleDbCommand(strsql9, conn9)

            myreader9 = cmd9.ExecuteReader

            ComboBox7.Items.Clear()

          

            While myreader9.Read()

                ComboBox7.Items.Add(myreader9("n1"))

                             Dim i, j As Int32

                For i = 0 To Me.ComboBox7.Items.Count - 2 Step 1

                    For j = Me.ComboBox7.Items.Count - 1 To i + 1 Step -1

                        If Me.ComboBox7.Items(i).ToString = Me.ComboBox7.Items(j).ToString Then

                            Me.ComboBox7.Items.RemoveAt(j)

                        End If

                    Next

                Next

                Dim i1, j1 As Int32

                For i1 = 0 To Me.ComboBox8.Items.Count - 2 Step 1

                    For j1 = Me.ComboBox8.Items.Count - 1 To i1 + 1 Step -1

                        If Me.ComboBox8.Items(i1).ToString = Me.ComboBox8.Items(j1).ToString Then

                            Me.ComboBox8.Items.RemoveAt(j1)

                        End If

                    Next

                Next

            End While

        Catch

        End Try