If your user provider is Identity Management, use api/v3/publisher/user/register (sets the user’s first name, last name, and email) or id/api/v1/publisher/identity/register (sets additional user parameters) instead of publisher/user/create.
Other providers or Identity Linking
If you call publisher/user/create with an email already in Piano's database, the endpoint returns the UID of that existing user (not the one in your request) and reset_password_email_sent=false. No user is created.
If you manage applications with Identity Linking Global mode and call publisher/user/create using an email registered in another application, the endpoint registers that existing user for the specified AID and the response contains reset_password_email_sent=true.