Register someone for a session return code 409

hi, about Register someone for a session
with : post
https://{defaultHost}/sessions/{id}/people

My question is :

When I register someone a second time with the same email, I get the following return code:

{
"errors": [
{
"title": "Create Identity Error",
"detail": "This person has already been invited to this event session.",
"code": "409",
"status": "conflict"
}
]
}

Is the 409 return code valid, since it's not mentioned in your API documentation?
Is the 409 return code only intended for this purpose (if a person is already known with this email, and therefore unable to register).
For this use case, is the return code always 409?

Thank you for your future clarifications.

best regards