The requested user is already being added or removed from this service instance concurrently with another API request. The last request will be rejected to avoid inconsistent state.
Possible Causes
You might be adding a new user to the service instance while the user with the same identity is being removed from another thread.
You might be submitting multiple concurrent requests to add a user with the same identity to the service instance.
You might be submitting multiple concurrent requests to delete the same user by SID or identity from the service instance.
Possible Solutions
Implement an operation retrier and repeat the failed user API requests after an interval of time, using an exponential backoff algorithm.
Review your application logic that caused the race condition when adding or removing users. Perhaps the conflicting operations are happening in a loop that could be avoided.