Estoy tratando de crear una serie de números aleatorios únicos (no duplicados) dentro de un rango definido por el usuario. He logrado crear los números aleatorios, pero estoy obteniendo valores duplicados. ¿Cómo puedo asegurarme de que los números aleatorios nunca serán un duplicado?

Sub GenerateCodesUser()
    Application.ScreenUpdating = False
    Worksheets("Users").Activate

    Dim MINNUMBER As Long
    Dim MAXNUMBER As Long

    MINNUMBER = 1000
    MAXNUMBER = 9999999

    Dim Row As Integer
    Dim Number As Long
    Dim high As Double
    Dim Low As Double
    Dim i As Integer

    If (CustomCodes.CardNumberMin.Value = "") Then
        MsgBox ("Fill Card Number Field!")
        Exit Sub
    ElseIf (CustomCodes.CardNumberMin.Value < MINNUMBER) Then
        MsgBox ("Card Number Value must be equal or higher then" & MINNUMBER)
        Exit Sub
    End If

    If (CustomCodes.CardNumberMax.Value = "") Then
        MsgBox ("Fill Card Number Field!")
        Exit Sub
    ElseIf (CustomCodes.CardNumberMax.Value > MAXNUMBER) Then
        MsgBox ("Card Number Value must be equal or higher then " & MAXNUMBER)
        Exit Sub
    End If

    Low = CustomCodes.CardNumberMin.Value
    high = CustomCodes.CardNumberMax.Value '<<< CHANGE AS DESIRED

    If (Low < 1000) Then
        'break
    End If

    For i = 1 To Cells(1, 1).End(xlToRight).Column
        If InStr(Cells(1, i), "CardNumber") Then
            Row = 2
            While Cells(Row, 1) <> 0
                Do
                    Number = ((high - Low + 1) * Rnd() + Low)
                Loop Until Number > Low
                Cells(Row, i) = Number
                Row = Row + 1
            Wend
        End If
    Next

    Application.ScreenUpdating = True
End Sub