Reminder Emails are sent twice

Hi there,
for quite some time now our service providers and customers receive the reminder emails twice.
The Appointments+ log only showed 1 email each.
We use the Plugin WP Mail SMTP to send email via SMTP. A few days ago I installed the Plugin WP Mail Logging which shows that two emails are sent each.
For some reason now the Appointments+ log is showing both emails as well.
We use Wordpress 4.7.4 and Appointments+ 2.1.0