Summary

1. If a user already exists and you try to create another user with the same email address it fails without showing an error.
2. If a user already exists and you try to create another user with the same email address though bulk create, it shows maxResults must be positive error.

Steps to Reproduce

1. Create a user on an instance with < 3 users
2. Try to create another user with the same email address

Expected Results

An error is thrown saying that email address is already taken.

Actual Results

No error is thrown at all.
If the user is added through bulk create, error maxResults must be positive is shown.

Notes

This is exacerbated by an 'inactive' user that doesn't appear on the list by default.
Suggest the error message to be meaningful, eg : The user already exists

Workaround

Create a user with a different email address or remove the original user.