Hi Richard,
Yesterday I was able to borrow a Windows laptop and using the list view in Outlook 2013 I was able (eventually) to delete the 12,000 duplicated entries, in stages. It took a long time for Outlook to load the calendar (around 15 minutes or so).
Now the calendar is fine in OWA and, after creating a new profile in Outlook 2015, that loads the pruned calendar as well.
Thanks for persisting with this. It would be nice to know what caused the problem in the first place, but at least I have a working calendar again.
Best Wishes,
Adam