i dont want to get duplicate value from database into combobox, i want my combobox should take every value only once and should not repeat it

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VB

i have a database table and it has some column with duplicate values , and i want to get that column value of the table in a combobox, but i want every value single time not repeatedly ... i tried but it is still giving me all values in the combobox:

Dim conn As New SqlConnection("Data Source=.\SQLEXPRESS;Initial Catalog=grade;Integrated Security=True;Pooling=False")
Dim cmd As SqlCommand
Dim Classes As New List(Of String)
Try
    conn.Open()
    Dim query As String = "SELECT Class FROM Class"
    cmd = New SqlCommand(query, conn)
    Dim rd As SqlDataReader = cmd.ExecuteReader(CommandBehavior.CloseConnection)
    While rd.Read()
        Classes.Add(rd("Class").ToString)
    End While
    rd.Close()
    conn.Close()

    Me.ComboClass.Items.Clear()
    Me.ComboClass.Items.AddRange(Classes.ToArray)

Catch ex As Exception
    MessageBox.Show(ex.ToString())


End Try