When you say he is receiving duplicates do you mean duplicates in the same folder? For instance I have two rules on my inbox, one sends emails to a folder with the name of the person sending it, and I have another folder where anything with TSM in the subject goes. So if an email arrives from that person with TSM in the subject I get a copy in each folder. Is this what is happening, or are they receiving duplicate copies both to the inbox?
If you have multiple duplicate items in outlook and you want to remove them, try Shoviv Outlook Duplicate Remover
tool for easier removal of duplicate items.