Calendar keeps showing notifications every 20-30 mins for repeated events

Thank you for posting your screen shot above. This clarifies what is happening.

You are dealing with clearing out the notification --- for the moment. But the Calendar app still has an alert on its agenda. So you keep getting alerts from Calendar, resulting in notifications to the Notification Center.

Open Calendar. Use Calendar's menu to go to Calendar > Preferences. Choose the Alerts tab. You will see something like this:

You may wish to uncheck both of the Notification Center boxes at the bottom.

You should also look for and respond to the messages that come directly from the Calendar app, which look like the screen shot I posted above. Respond to them by clicking "Close".

You can read this article : macOS Sierra: Receive or stop notifications

: macOS Sierra: Notifications System Preferences

You told us the events repeat daily.

You failed to tell us when the alerts occur (e.g., 15 minutes before the event).

You also failed to tell us what action you take when the alert appears. Do you close the alert? Do you snooze the alert?

The actions I see available are "close" and "snooze". You said you "dismiss" them. So perhaps that means you did not actually close out the alert.