Hi @casper!
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.
https://fusionauth.io/docs/v1/tech/apis/users/#import-users
validateDbConstraints [Boolean] OPTIONAL Defaults to false
Set this value to true in order to perform additional validation of the request.
The import request is intended to be used to populate the initial set of users, this means FusionAuth does not expect to find duplicate users in the database. If a duplicate is encountered a 500 will be returned without this additional validation.
If you intend to use this API with existing users in FusionAuth set this value to true to request additional validation be performed on the input request and a 400 response will be returned with JSON body indicating the duplicate values encountered.
Setting this value to true will dramatically decrease the performance of this request. If importing large numbers of users in a single request you may need to increase request timeouts to ensure this request does not timeout before it has completed.
I hope this helps,
Josh
