I had this exact problem a few weeks ago and started researching stop duplicate emails in Mac Outlook to find a proper solution. What I found is that duplicate emails are often caused by sync conflicts between Outlook and the email server. If Outlook cannot confirm whether a message has been downloaded, it may download it again just to be safe. Using the same account on multiple devices can also increase the chances of duplication. In my situation, some old email rules were also copying messages into multiple folders. This not only cluttered the inbox but also slowed down Outlook performance. I cleaned up the rules and adjusted the sync settings, which helped a lot. Now the inbox is much more organized.