@Mark Muyskens: According to this post in your aforementioned thread, this issue is NOT fixed in 17.8....
A DKIM-Signature header is duplicated when an email is sent to several recipients
I just upgraded to the early adopters version 17.8.11 Update 1.
For me this issues is still not fixed. Sending an eMail to multiple receipients still results in as many DKIM signatures as are receipients in the eMail. Such emails are not accepted by major mail providers.
As @IgorG pointed out...