duplicate mails in Outlook(office 365)

Clarifications

Make sure both do not have a filter/rule setup.

Also, are they both setup as a delegate for someone or perhaps to each other?

I also assume this happens if just one person sends them an e-mail? Or is it happening when it's part of a multi recipient e-mail?

Hi,

You should try to use Kernel for Outlook Duplicates which permanently removes duplicate items of Outlook mailbox. It searches and deletes duplicate entries of an Outlook account on different parameters such as senders email text, senders name, internet header, receiver name and others.

You can get more Information about this tool from here http://www.outlookduplicateremover.org/