Same newsletter sent many times

Lately I’m getting few people that unsubscribe, and today I noticed that for a couple of subscribers, the auto newsletter when out more than once.
The newsletter is set up to go out immediately after someone subscribes to the mailing list. Only ONCE.
I have one profile that get 5 times the same email (following the stats). However, the plugin says Subscriber opened 2 of 3 emails.
But I can clearly see that 5 identical emails went out.
Now I’m a bit worried that I’m spamming my subscriber with duplicated emails.
Please let me know if you need anything.