Using the New Outlook for Mac for personal email and calendar
Since Gmail works correctly and only AOL is creating duplicates, the issue is likely related to the AOL IMAP sync settings rather than Outlook itself. Check whether the AOL account has been added more than once in Outlook, and verify that AOL isn't forwarding messages to itself or another connected account. You could also try removing the AOL account completely, clearing the Outlook cache, and then adding it back as a fresh IMAP account. If the duplicates appear only in Outlook and not in AOL Webmail, that strongly points to a sync issue between Outlook and AOL.