Ok - every time i start Duplicati GUI (web) after some Days for the first time i realise that my backups are not running in the meantime. But my duplicati.service (systemd) is always on also after a reboot.
So - just after starting this duplicati app i can see the last time duplicaty made a backup and it runs a new one.
I asumed duplicati runs in background when the service is startet. sorry my english.