Hi Emily,
I’m sorry to hear you have experienced difficulties. In the first instance, can you confirm which version you upgraded from?
I also wonder if you have any customizations in place (such as template overrides) or anything of that order that you could quickly and temporarily remove to see if they might be impacting?
The reason I ask is that, in the past, the duplicates have been just that: identical but separate copies of the event. When I look at November I see that it is more a case of the same event being linked to multiple times – so there’s a subtle difference there.
I see that others have had this problem with past upgrades. I am not able to shut down the theme – the site’s owners are panicking about all of this.
I can definitely appreciate that.
Given the slightly odd twist to the problem in your case I do think it’s worth eliminating the potential for a conflict here. Are you in a position to test things out on a separate test installation? Plugins like Duplicator and various other tools would make it easy to copy the existing site to a test installation (which could simply live in a subdirectory, ie yourdomain.com/testwp) and you could then troubleshoot there without fear of impacting the live site.
Is that a possibility?