Short update: we got deeper into the matter and found out that our problem was connected to nameservers that became suddenly unresponsive and needed to be replaced for our domain.
We have the exact same problem since the beginning of july. In our case it seems to primarily affect eMail aliases. Mainly we had problems with eMails from Amazon or eBay that are being handled via an alias of our main eMail-address. These mails are being duplicated up to 10-15 times. We also...