Subscribe by email send multiple emails

Hi there.

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.