Calendar sending multiple e-mail alerts for events.

When I put an event on Calendar that concerns something I need to do but it's ok if I do it after the event alert disappears from my desktop. To give myself a permanent reminder, I add an alert to send an e-mail to myself. When the e-mail arrives, Mail processes it using a rule that puts it into a reminder mailbox. It used to work great, but after I upgraded to macOS v. 10.13.1, Calendar (v. 10.0 (2194)) started sending multiple (up to 8 or 9) e-mails. It still works, but I'm getting a gazillion extra reminders that I have to delete.