Why do I get duplicate e-mails from unattended-upgrades?

I have no idea where to look for this. Since the upgrade from Ubuntu Server 14.04 to 16.04 I consistently get every single e-mail from unattended-upgrade exactly twice. They have the same content and headers and even the same message ID but one is delivered some seconds later. Where can I find the reason for this? Has anybody seen this, too?