[FEATURE] Don't send notification emails when already delivered to Slack

**Is your feature request related to a problem? Please describe.**
It's awesome to receive error notifications in Slack, but I wish we wouldn't also receive the same duplicate notification via email if that notification has already been delivered to Slack. We make a point to respond to all emails & Slack notifications, so this doubles the work.

**Describe the solution you'd like**
An option to not send notifications emails if they have been successfully delivered via Slack (or do this automatically, but idk what other customers may prefer). But if the Slack delivery fails for some reason, then we should receive the email as a backup notification.

**Do you have a workaround?**
Yes. This is just a nice-to-have/quality of life improvement.

**Additional context**