Symptoms
There may be duplicate default folders (e.g., Junk Email, Deleted Items, Sent Items, etc.) as well as duplicate Calendars in your TechMail account showing up either through Outlook on the web, on your mobile device, or in Outlook.
Cause
Usually, this situation happens when your email client software abruptly quits/crashes or for another reason becomes out of sync with the TechMail servers. With the situation on the calendar, the duplication creates a calendar with all of the correct appointments and then a secondary calendar that is blank.
Unfortunately, the secondary blank calendar becomes the default calendar in your TechMail account.
Resolution
NOTE: Default Exchange Online folder names (e.g., Calendar, Inbox, Junk Email, etc.) cannot be deleted. Because the default calendar folder has likely changed from what it was before, the calendar items should be manually moved from the previous location to the new default calendar folder (named "Calendar").
First, run the /resetfoldernames command-line switch to clean up any problems with the folder names and structure.
Finally, delete the duplicate calendar folder.
Perform the following steps on a Windows computer with Outlook installed.
1) Close Outlook.
2) Run the /resetfoldernames command-line switch. (Instructions)
3) Go to the calendar folder in Outlook where your items are currently stored.
4) Press Ctrl+Shift+F. An Advanced Find window will pop up.
5) Select Appointments and Meetings in the "Look for" drop-down menu.
6) Click Browse and select the calendar that has the data you want to keep.
7) Click Find Now.
8) Once the search is complete, select all of the entries.
9) Right-click and choose Move to folder.
CAUTION: Always move calendar contents; never use the "Copy" function.
10) Choose the default Outlook calendar (named "Calendar").
11) Once you are certain that all the items are removed from the duplicate calendar that you wish to delete, you may right-click on it and delete it.
OUTCOME
The duplication of TechMail calendars will be resolved without loss of data.