Calendar entries are duplicating

This can happen in some cases where synching and accessing your mailbox and/or Calendar on multiple mobile devices is involved.

If not all the items are duplicated, it usually only affects meetings which got updated and recurring items.

Locating the source

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. 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.

In either case, you should start with disabling all the syncs that have been set up. This can be for instance;

• A sync application that directly syncs with your mobile phone.
• An application or add-in which syncs your Calendar with an Internet Calendar (such as Google Workspace Sync for Microsoft Outlook).
• A mobile device which syncs directly with the mailbox/calendar via Exchange Active Sync (turn off the device).

In other words; use plain vanilla Outlook. 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.

You can now turn back on syncing and test it. 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.

In addition, make sure you are using the latest version of whatever sync application or add-in that you may use with Outlook.

Possible causes - background info

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 those cases individual items are being created on the other device which could be seen as new items during the next sync process and thus cause duplicates.

For meeting invitations and updates, the origin usually lies with having your mobile device configured to collect only your email directly over-the-air and the mobile device also being capable of processing incoming meeting requests.

As it processes these meeting requests, it will place it in your calendar on the mobile device. As you’ve also received this invitation via Outlook, it will have been added to your Outlook Calendar as well.

When you would now sync your calendar via a desktop sync application, the sync application will not recognize these items being the same but both new (as they were created differently and never have been synced before) and keeps and syncs both items resulting in a duplicate.

Especially this last scenario is quite typical when using your smartphone in combination with a POP3 or IMAP account and also a 3rd party sync tool for your Calendar.

One solution would be to use a fully hosted account with calendar syncing support (like Exchange or Outlook.com) and only sync your mobile device over-the-air.

Duplicate Holidays

When the holidays show up multiple times in your Calendar, they usually have been imported multiple times via;

• File-> Options-> Calendar-> section Calendar options-> button: Add Holidays…

Unfortunately, this process doesn’t update or just add newly added ones but duplicates them.

Getting rid of these duplicates and updating your Holidays correctly is discussed in depth here; Holiday updates for the Outlook Calendar.

Duplicate Birthdays and Anniversaries

Birthday and Anniversary  events for Contacts are automatically added to your Calendar when you define the birthday or anniversary for your contact.

In some cases, the connection between these events on the calendar and the contact is lost and updating any contact info will add another Birthday or Anniversary Event to your Calendar for that contact.

Cleaning up the duplicates

Once you’ve found the source of the duplications, cleaning up the duplicates can be done in one of the following ways;

• Manually – Subject based
  Switch your Calendar to the All Appointments view via the View menu or tab.
  You can now more easily identify and select which items have been duplicated and delete them manually.
• Manually – Date based
  As the creation of the duplicates all happened at the same time due to the sync process, then adding the Modified Date column to your view and sorting by it should put all the duplicates together and allow for easy recognition and deletion.
• Automatically via an add-in
  If you have a lot of items or when sorting by date doesn’t work, then the manual process might not be practical. In that case you can use a third party Duplicate Remover Tool.