If you are using a tracking pixel on emails, then Sendgrid generates an Open event every time the email is read or accessed by the end user. This is just a by product of how tracking pixels work and have been implemented in Sendgrid. However, from a Fivetran users perspective, a single email can generate very many event, that all count against MAR. 

Sendgrid talk about Unique Opened vs Opened events on their site - https://docs.sendgrid.com/glossary/opens

and the types of events

https://sendgrid.com/blog/the-nine-events-of-email/

The request is to see if there is a way to just capture the first Open event and sent that to the connector rather than send all Open events to the connector. Maybe this could be an option as part of the connector setup.