[VBA]

Private Sub Worksheet_Change(ByVal Target As Range)
Const WS_RANGE As String = "A1:B100"

If Not Intersect(Target, Me.Range(WS_RANGE)) Is Nothing Then
Application.ScreenUpdating = False

If Application.CountIf(Me.Range(WS_RANGE), Target.Value2) > 1 Then

If MsgBox("Accept Duplicate?", vbYesNo, "RepeatedValues") = vbYes Then

Target.Interior.ColorIndex = 40
Else

Target.Value2 = ""
End If
Else

Target.Interior.ColorIndex = xlNone
End If

Application.ScreenUpdating = True
End If
End Sub
[/VBA]