Finding duplicate across many sheets

Hello hero96559,

You need to be more specific about what data you want to check. You did not include any examples in either your post or the attached workbook.

Hello hero96559,

Do you want to validate entries in a specific column or columns across all sheets?
Checking all sheets anytime any cell is changed will slow your data entry down.
I assume headers would not be included in the validation, correct?

Hello hero96559,

Are you only interested in being notified if the serial number is duplicated?
Is there only one sheet used for input?

Hello hero96559,

The solution I am working on is creating a "Master List" worksheet of serial numbers. Whenever an entry is made in any cell then it can be checked quickly for duplication. So, this process is going to take awhile to complete. It may be a few hours before I finish.

Hello hero96559,

The attached workbook validates any input in any cell against the "Serial Number" list in the "Master List" worksheet. There is an issue I need to workout. Serial numbers with a leading zero have to be input as a text value or the leading zero will not be retained. Since all the data in the "Serial Number" list is text. The other issue is how to update the "Master List". Perhaps you have some ideas about that. Otherwise, it works well.

Here is the macro to look up the serial numbers.

Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)

    Dim Match As Range
    Dim Rng As Range
    Dim RngEnd As Range
    Dim SN As Variant
    
        If Target.Cells.Count > 1 Then Exit Sub
        If Target.Value = "" Then Exit Sub
        
        Application.Calculation = xlCalculationManual
        Application.EnableEvents = False
    
        With Worksheets("Master List")
            Set Rng = .Range("A2")
            Set RngEnd = .Cells(Rows.Count, "A").End(xlUp)
            Set Rng = .Range(Rng, RngEnd)
            For Each SN In Rng.Value
                If SN = Target.Text Then
                    MsgBox "The Serial Number '" & Target.Text & "' has already been Entered.", vbOKOnly + vbExclamation
                    Exit For
                End If
            Next SN
        End With
        
        Application.Calculation = xlCalculationAutomatic
        Application.EnableEvents = True
        

End Sub

Hello hero96559,

Everything is working fine with no problems or not?

Dear Leith

You can