Hi Thijs,
I can’t see the inside of your system, but I have one quick idea.
Do you have a lot of HTML in your email template (which they tend to call the "report" for some reason)? If so, the recipient may be getting a plain text version and an HTML version, which aren’t identical, but pretty similar. Some email clients will divide these into 2 separate emails, others may combine them into 1, I think.
If that’s the case, you could try a plain text "report" template, and see what happens.
This might be a longshot, though. Maybe it’s something on the server. You could ask tech support for your host to look at the email logs to see what’s happening.
Good luck, Dave