Find/Remove Duplicates & Keep Original

Finding it difficult to remove duplicates and keep the original entry. "Remove Duplicates" does not work.

1 person can make multiple complaints under one booking ref with a different Type/Issue Desc./Subject/Cost Type/Unit Cost. I need to keep all original cases and then highlight/remove duplicates

I tried in colom AG "Dup2" marked in red, but I failed...

Your formula works better like this:
=IF(COUNTIF($AF$2:$AF2,$AF2)>1,"Duplicate",""), and copy down.

The #VALUE error you get is because the number of characters in COUNTIF criterium is over 255.
Maybe the link below will help you get around it.

https://stackoverflow.com/questions/...google_rich_qa