Importer for Contacts picks up where Apple Contacts left off by importing Excel files, CSV files, tab-delimited files, and vCards into "Contacts" with more options.
As a bonus, it can also convert CSV and Excel files to vCard files directly without the need to import them first. The app is also able to restore Address Book groups from vCards.
Importer for Contacts never does overwrite or delete existing contacts. Instead, old duplicates will be added to a group in Contacts, making it easy for you to remove them manually if the import worked right.