Event already processed

When I get an invite to an event from a different user, not to that email address, that is set in the Nextcloud settings, but to different email address, I get the error message in Thunderbird, that the message has already been processed.

Has somebody else seen this behavior before and is there a workaround for this?

This seems as it would be a NextCloud issue.

Their support forum is at https://help.nextcloud.com