In a message of Wed, 28 Oct 2015 10:22:50 +0100, Marco Stoecker writes:
But now I have no glue how to configure on the ISP side or postfix(relayhost) or mailman to avoid such duplicates. Any assistance possible here in this mailing list?
There is a fetchmail-users mailing list here: https://lists.sourceforge.net/lists/listinfo/fetchmail-users if you need to ask about how to configure fetchmail. I am not sure that is your problem, though.