Simple Copy/Paste Macro with Duplicate Prevention.

Hi, and welcome to the forum.

The way I usually trap this situation is to use a MATCH() function. So for example in your case in another cell named say 'Check" add

Formula:

=IF(ISERROR(MATCH(L12,Output!A:A,False)),"OK","Duplicate")

Then your macro is

Sub GENERATOR()

   IF Range("Check") = "Duplicate" Then
      MsgBox "There is a duplicate on the Output sheet. This record will not be added."
      Exit Sub
   End If

   Range("L12").Copy
   Sheets("OUTPUT").Range("A" & Rows.Count).End(xlUp).Offset(1, 0).PasteSpecial(xlPasteValues)

End Sub

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