@bertnl
As far as I know, an email will be sent when an "alarm event" starts and when the "alarm event" stopped ( End). So yes for every event, 2 emails will be send.
In the email message, you will see what kind of alarm event has been triggered (Tripwire , Intrusion , Face Detection , Motion Detection ,etc...).
Only one image (jpg) is added as an attachment to the first or second email depending on the alarm event type.
So if you only receive alarm events of type Motion Detection, you could disable "send email" in the NVR at ALARM, Video detection, Motion detection, MORE.
Check all setting for all other cameras, from which you need or don't need an email.