Since I started a subnet_scan every device appeared 4 up to 7 times in the Database.
How can I solve this Problem?
Your devices are not sending enough details for Open-AudIT to match them AND you have the config item "discovery_ip_match" set to 'n'.
Make sure to add credentials using menu -> Admin -> Credentials.
Run a few test discoveries on SINGLE devices utilising the 'debug' option on the discovery form. Wait for this page to complete (it will finish after everything on that device has been run so it might take a minute). Read through the output and you'll see why the device has not responded (most likely bad credentials or target device firewall, etc).
You might also find some useful info on this wiki page - Target Client Configuration.