Daily digest e-mail sent twice with nearly the same posts


sometimes the Subscribe by Email plugin sends two mails to all users in daily digest mode. First with all new posts (today seven), then 20 minutes later with the oldest five of them.
Today it was the second time, that this occurs. It's not every day. Maybe only if there are more than 5 new posts per day.
In the Emails Log page I can see two entries for today with the same time and the same number of receivers (60), all successful. Max numbers of receivers is configured to 80, so all e-mails sould be sent at once.

Any ideas what the cause could be?