..submitting the same site many times by refreshing your page.
I mean that it still inserts in the database each time you refresh the page.
Could you test it ? I think it's because of '$sql = $wpdb->prepare("select website_id from ".$table_name." where website_url = %s.. ' in a function called 'doWeAlreadyHaveThisSubmission' in the file persistence/UbLinkpartnerMapper.php
It's due to the prepared statement which can be sometimes concerned as a cache.
Do you see what i mean ?
May be $wpdb->query would be better than $wpdb->prepare ?