Post Notification Email send twice to some recipients

Hello,

I switched to your plug-in a couple of days ago and used it for the first time to send out post notifications to my +3k subscribers. It worked fine, however some of my subscribers informed me, that they received the email twice.

What I can say is, that…

1. I have 3 addresses myself in the list of subscribers and did not receive the email twice.

2. I received 2 auto-responders from the same recipient with different timestamps and ID (which they add to their auto-reply subject line).

3. Several recipients wrote me, that they received my email twice.

Any suggestions? Thank you.