Often the cause of the duplication for recurring items is the fact that the mobile device or synching application that you are using does not support Outlook’s recurrence patters or exceptions to recurring items. In addition, make sure you are using the latest version of whatever sync application or add-in that you may use with Outlook. If you are using multiple sync methods, only use 1 at a time and only start combining after you’ve verified that none of the syncs are creating duplicates of their own. You can now turn back on syncing and test it. Now create a new recurring item or a meeting items and make any modification after saving or sending and see if you still are getting duplicates.

In other words use plain vanilla Outlook.

A mobile device which syncs directly with the mailbox/calendar via Exchange Active Sync (turn off the device).

An application or add-in which syncs your Calendar with an Internet Calendar (such as Google Workspace Sync for Microsoft Outlook).

A sync application that directly syncs with your mobile phone.

In either case, you should start with disabling all the syncs that have been set up. As said, most duplications appear to be happening when you receive an update for a meeting or when you modify a recurring item see if this is also the case for you. How to locate the source depends a bit on if you are able to reproduce the duplication and how many devices you are synching with. If not all the items are duplicated, it usually only affects meetings which got updated and recurring items.

This can happen in some cases where synching and accessing your mailbox and/or Calendar on multiple mobile devices is involved. How did this happen and how can I troubleshoot and fix this? Looking at my Calendar, I see that several (not all) of my appointments and meetings are suddenly showing up multiple times (some even more than 5 times!).

Delete duplicates in outlook calendar