Subscribe2 often sends two (or more) e-mails to some of my subscribers. It's causing them to complain, causing some to unsubscribe, and recently caused my account host to threaten to shut me down for exceeding their hourly limit. I use WPMailQueue, with a tight throttle, but this duplication is still causing the system to break throttle.
In fact, this has been going on for years. And it has to stop, or else I'll have to find another solution, or stop sending e-mails altogether.