Only time will tell ...
@tonyj3 your lucky. As I mentioned previously, I had an event which I set up without an end date. I started getting 2 reminders for this event so I deleted the event and set up a new event. I now get 3 reminders - one for the new event and (still) 2 for the event which I deleted. Bizarre!
@tonyj3 and @delphinium9
I've updated the Feedback thread - see https://community.bt.com/t5/Email/BT-Webmail-Feedback-v2-post-16th-April-release/td-p/2376642/page/3... , post 347.
A deleted pre-migration event has stopped generating reminders (for the moment at least).
A post-migration event which was generating single reminders now generates duplicates.
The Lord taketh away and the Lord giveth back.
You're (a bit) luckier than I am......I'm still getting email reminders from pre-migration events as well as duplicates for post-migration events. The pre-migration was set up without a determined end-date which may explain why I'm still getting reminders even though I deleted the event itself.