This article refers to MailPoet 2
If you're looking for MailPoet 3 articles, please go to MailPoet 3 Knowledge Base
If your MailPoet plugin is sending newsletter duplicates to your subscribers, your website is probably getting high levels of traffic and your MailPoet Cron option is deactivated.
The path to enable MailPoet Cron is at Settings page > Advanced tab > Geeky Options link.
Want to understand what a "Cron" is? Read here: Understanding the Cron