Apologies for the delay David, since the post we manually deleted the duplicates and the problem has not recurred, so it is a bit of a mystery what caused it originally. I appreciate it is a bit late but I have answered your questions based on my recollection, but I guess until this happens again we cannt take this much further.
Currently, multiple factors such as the client error, the bad calendar entry can lead to the problem. To narrow it down, could you please provide the following information?
Based on my experience, the screenshot you provide is the calendar page of Outlook 2013. Does the issue occur when the calendar was shared with Outlook on Mac?
[NT] Yes
2. If so, which Outlook client such as Outlook 2011, Outlook for Mac did the user use to share the calendar?
[NT] Outlook 2011
3. You mentioned “The original meeting was a recurring meeting”, does the issue exist if the same user shares a “non-recurring” calendar?
[NT] No, only recurring meetings
4. When checking the shared calendars with OWA, Is the same calendar duplicate?
[NT] As far as I am aware this was the case, unfortunately the duplicates have now been manually removed and not seen since
5. Could you help confirm the affected range, say, how many users, how many calendars are affected?
[NT] There were at least 2 people impacted
6. To confirm whether that Outlook client error is the root cause, please remove the connection between Outlook and the Office 365 account and test the issue with OWA only.
[NT] II am afraid we cant try this now as the duplicates were manually deleted.