If I understand correctly it would seem as if the following should work:
Formula:
=IF(AP4="","",IF(COUNTIF($AP:$AP,AP4)>1,IF(COUNTIF($AP$2:AP4,AP4)=1,AP4,""),""))
That said it would help us to help you if you would upload a sample of the spreadsheet from which the screen shot in post #1 was taken. Manually place the values you want in column AQ so that we can attempt to write a formula to automate those results.
To upload a sample workbook click on the GO ADVANCED button below the Quick Reply window and then scroll down to Manage Attachments to open the upload window.
Let us know if you have any questions.
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