In order to only insert new values, I'd recommend you to tag each row of data with a unique identifier. Then insert a logic that checks whether that identifier already exists in the DB or not.

See if the below article helps -

https://gbamezai.medium.com/how-to-copy-data-from-sharepoint-list-to-sql-using-power-automate-conditional-data-load-651d7520efd4?source=your_stories_page-------------------------------------