Is there an easy way to attach a macro that will run when a sheet is open so that if a value is entered say in A10 and the value already exisst in the range A1:A9 then a message warning that it's a duplicate will appear?
Try this: right click the sheet tab, select View Code and paste in
Code:
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Column = 1 Then
If WorksheetFunction.CountIf(Columns("A"), Target.Value) > 1 Then MsgBox "Duplicate!", vbInformation
End If
End Sub
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