Importer for Contacts
For Mac
Convert Excel and CSV files to vCards with much more options than Apple Contacts does offer.
Importer for Contacts Support FAQ
After importing contacts into a macOS Server CardDAV account, all imported contacts and groups were duplicated!
It is not known why this behavior occurs, apparently it is a bug in macOS Server. Use the vCard conversion feature of the app instead.
UPDATE: Apple seems to have fixed this problem in macOS 10.13.x / macOS Server 5.5