Duplicate entries in Outlook Calendar can occur due to inconsistencies when importing, exporting or syncing your calendar data across multiple devices. You can prevent duplicates from appearing in your calendar by having the proper settings in place when using the application's Import/Export Wizard. Furthermore, you can avoid creating duplicates through syncing by ensuring that you are using the appropriate programs to sync your Outlook Calendar with the calendars on your mobile devices.
Settings in the Import/Export Wizard
When you are using the Import/Export Wizard to update your Outlook calendar with data from a file, you can choose if you want Outlook to overwrite your old calendar entries with the updated data or save duplicate entries alongside the originals. The advantage of saving duplicates is that you can compare the new and old data to make sure that nothing important was overwritten. However, if you are certain that you want your calendar entries to be overwritten by the new data, select "Do Not Import Duplicates" before you start importing the new file.
Syncing Calendars Between Android or iOS Devices
If you use a calendar syncing app for your mobile device, consider disabling it to determine if it is the cause of your calendar's duplicate entries. If the program you are using is not Windows-, Google- or iOS-based, Outlook may have difficulty communicating with the program, resulting in syncing issues. While it may take time, using iTunes or Google Apps to sync your Outlook calendar with the calendars on your Android or iOS device can help ensure that your data syncs properly. In addition, maintaining your calendar on Outlook.com and syncing it with your other accounts on the Web can be a faster and more reliable way to transfer your information back to your physical devices.