I really don't like Outlook notifications. So I created a flow that sends me a message on Slack when an event is 15 minutes away.
It works well. Too well. Approximately 30 seconds, it sends me another reminder. I can't find any logic to space out the polling interval for the calendar check, or any check to see if I was already notified.
Why do you want to space out the polling interval? Not sure what you are trying to acheive here. If you add another flow below slack flow you can acess slack properties such as Post time, Success Result etc.
Before the flow post message add a HTTP flow and use the Slack API (https://slack.com/api/channels.history) to get the old messages from Slack. Add an if condition to check the HTTP GET message body contains calendar subject or not. If nothing found then add the message to Slack.
I haven't tried this. Let me know how it goes. Good luck.