Welcome to the community. This is working as designed. The import API is currently designed to only write new records to the database and not check if the record is already duplicated. This is partly for performance reasons. It maybe be possible to write a script to check existing records, and then only import "new" users based on that feedback, but we do not have any current documented cases of this.
There is an option to get additional information about what is failing on your import.
I hope this helps,