Create Resource User API Error - A record with the value FUSION_HCM <PersonID> already exists.

Summary:

We have a process to create a resource user for existing workers in HCM via Create Resource User API. This has been working but some of the users can't be created due to this error:

A record with the value FUSION_HCM,
300000XXXXXXXX already exists. Enter a unique value.

Can you please advise what might have been causing this issue? All the other successful created resource users also had an existing HCM worker record so we can't identify why it's only happening to some records.

Content (please ensure you mask any confidential information)
API endpoint: POST

https://<fusion>oraclecloud.com:443/crmRestApi/resources/11.13.18.05/resourceUsers