Double sendings

Hi, I have noticed for the 7 month of usage of your plugin, that some days it sends 2 mail newsletters in stead of normally just one. I have set it up to collect a list of the latest 15 posts and send them at 4 p.m. to my subscribers.
The double newsletters are 100% similar.
No cron jobs are activated on the server and are not used.
It’s disturbing. How can this issue be solved?

Thanks.