Been using this plugin for month's but updated the plugin also this week on this site.
Only other plugins installed also this week was:
Browser Caching with .htaccess - new installed
Easy Pricing Tables by Fatcat Apps - new installed
Redis Object Cache - new installed
WP Hummingbird - new installed
WP Smush Pro - new installed
WPMU DEV Dashboard - new installed
The newsletter have about 9000 subscribers but send out 59833 newsletter this week. On my two test emails I received between 5 and 9 duplicate emails.