Invitee is in inviter tenant: Azure AD Invitations Endpoint Error - php

Question

I want to call the Invitations API to invite users to sign in via Microsoft Graph API.
My code works for most of the emails that I call the API on, however for some emails, I get this error:
{
"error": {
"code": "BadRequest",
"message": "Invitee is in inviter tenant",
"innerError": {
"request-id": <request id>,
"date": 2019-06-03T05:51:21
}
}
}
Any idea why this issue occurs?
To be clear, when I first called the API on this email, the invitation email was sent successfully. However, after a few times of testing, I get this error.
I initially suspected that clicking on the 'Get Started' button in the email caused on this error and tested this hypothesis on an Outlook email. However, after clicking on the 'Get Started' button for the test email and calling the API again, the invitation email is still sent as per normal.
My code snippet is as follows:
$curlStat = curl_init();
$feedURL = 'https://graph.microsoft.com/v1.0/invitations';
$data_body = Array(
'invitedUserEmailAddress' => <test-email>,
'inviteRedirectUrl' => <my-redirect-url>,
'sendInvitationMessage' => true
);
$headers = array(
"Authorization: Bearer " . <my-access-token>,
"Content-Type: application/json"
);
$data_body = json_encode($data_body);
curl_setopt($curlStat, CURLOPT_URL, $feedURL);
curl_setopt($curlStat, CURLOPT_POST, true);
curl_setopt($curlStat, CURLOPT_HTTPHEADER, $headers);
curl_setopt($curlStat, CURLOPT_POSTFIELDS, $data_body);
curl_setopt($curlStat, CURLOPT_RETURNTRANSFER, true);
$emailOutput = curl_exec($curlStat);
curl_close($curlStat);

Answer 1

I'm aware of two conditions that cause this (or similar) errors to fire:
You're attempting to invite someone with an address from one of the verified domains. In other words, if your tenant lists company.onmicrosoft.com and company.com as verified domains, attempting to invite an address ending in #company.onmicrosoft.com or #company.com will fail.
The external email address you supplied already exists in the tenant (typically because they've already accepted a previous invitation).
Both of these boil down to the same root cause: you cannot send an invitation to an email address that has already been assigned to an existing user.
As for why you could send an invitation immediately after accepting a prior invitation, this is likely just a race condition. It takes a few moments for the invitation to get processed, the user record created, and the changes to propagate across your AAD tenant. Try waiting for 1 minute after accepting the invite and I suspect you'll consistently see this error.