I found the problem. It isn't only possible to define journal rules. It is possible to define a Journal receipient also on the data base.
[PS] C:\Windows\system32>Get-MailboxDatabase "mailbox database 2016" | fl *journ* JournalRecipient : mydom.loc/Group/Users/GroupAgain/Administrator
I found the Setting also in the Management tool for the database. After Clearing it no more emails are sent as a duplicate to my Admin Mailbox.