Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)
If Not Intersect(Target, Range("A10:A50")) Is Nothing Then
For Each it In Sheets
For Each it1 In it.Range("A10:A50").SpecialCells(2, 1)
If Target.Value = it1.Value And Target.Address(, , , True) <> it1.Address(, , , True) Then MsgBox "double in " & it1.Address(, , , True), , Target.Address(, , , True)
Next
Next
End If
End Sub