Reminder e-mail sent multiple times

We are using the feature for a reminder e-mail to our customers to leave a product review. We set the scheduled time to 14 days. We use WP Mail SMTP as sender plugin with sendgrid. We noticed that about 20 % of the mails are sent multiple times. Some are sent twice but it goes up to even 18 times.

I can only guess but is it possible that the scheduler does not processes the time correctly so that it does not recognize previously created or sent reminder e-mails? If not what could cause this issue?