Ok, I have a better idea now...
a- an envelope is created for each RCPT TO in a transaction...
b- ... and additional envelopes may be created by aliases _during_ that RCPT TO
Thanks a ton for those two pointers, I'll investigate and write what I
can figure out.
You correctly guessed a), turns out that my MUA (msmtp, but only when
triggered by mutt) somehow doubles all RCPT TO addresses (but not
always, as said). I'm not sure at the moment why, but I can exclude
OpenSMTPd from being the culprit.
I guess this was already going on for months, but previously used MSAs
weren't under my control, so I never noticed it. Oops.
Thanks again for your precise answer which pointed me into the right
direction, and sorry that I didn't check it more thoroughly from the get