google shortner api is not giving new urls - api

Question

I have set up for google api shortner on my website and i have it to get new shortned urls when ever someone places there url on my site. This is the code i have:
if($REQUEST_METHOD=="POST"){
if(isset($add)){
$longUrl = $url;
$apiKey = 'My api key';
$postData = array('longUrl' => $longUrl, 'key' => $apiKey);
$jsonData = json_encode($postData);
$curlObj = curl_init();
curl_setopt($curlObj, CURLOPT_URL, 'https://www.googleapis.com/urlshortener/v1/url');
curl_setopt($curlObj, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($curlObj, CURLOPT_SSL_VERIFYPEER, 0);
curl_setopt($curlObj, CURLOPT_HEADER, 0);
curl_setopt($curlObj, CURLOPT_HTTPHEADER, array('Content-type:application/json'));
curl_setopt($curlObj, CURLOPT_POST, 1);
curl_setopt($curlObj, CURLOPT_POSTFIELDS, $jsonData);
$response = curl_exec($curlObj);
//change the response json string to object
$json = json_decode($response);
curl_close($curlObj);
in "my api key" i do have a api key and i do place it there and i do have shortner api enable in google. To echo the shortened url i use:
<?print $json->id;?>
which works fine. My problem is that if someone places the same url twice, it will give them the same shortned url and not a new unique one. Would any of you know what could be causing this problem? Is there something wrong in my coding?
Any help is much appreciated. Thank you

Answer 1

This is interesting situation I just ran into. If you authenticate using your api key, then indeed you get the same shortened url or id for every long url you pass in. That's actually what I want for my use case!
But if you authenticate using OAUTH then you get a new unique id for every long url you pass in, even when they are the same.
You can see this behavior in the api explorer when you toggle on and off OAUTH authentication:
https://developers.google.com/url-shortener/v1/url/insert
I don't see this documented anywhere, so its' hard to know whether this is a feature or a bug, or what we can expect in the future.