UPDATE:
it works when i use gromox-eml2mt with the --ical flag
sudo gromox-eml2mt --ical feiertage.ics | sudo gromox-mt2exm -u user@domain.tld -B Calendar/Feiertage
or gromox-ical2mt command
sudo gromox-ical2mt feiertage.ics | sudo gromox-mt2exm -u user@domain.tld -B Calendar/Feiertage
But then the -B option with the specified path will be ignored and everything is importet in the main Calendar folder.
For the Problem with the duplicates i found a solution. Befor import the downloaded ics file i empty the folder with running
sudo gromox-mbop -u user@domain.tld emptyfld Calendar/Feiertage
This would be a good solution for me. I hope maybe someone can fix this problem with the ignored -B option for the gromox-ical2mt command.