Older push notifications are being sent repeatedly from APNS - ios

Question

I have an iPhone application that receives push notifications sent from a PHP server. I implemented the tutorial on Raywenderlich for this task.
I am sending push notifications for schedules in user's calender and some users have reported that the push notifications reminding user about a particular schedule gets sent repeatedly.
To be precise, when first notification comes it behaves normally, but when second comes it displays older notification too. The time of older notification is old as well. Only the current notification shows Now, the older notifications show some something like 10 hours ago. Then third notification shows 3 notifications and so on.
Someone told me to use APNSPHP library and set a unique custom identifier on every notification but that didn't help either.
What I am doing is that I am sending several push notifications (albeit to different users) over a single connection to APNS. The payload contains a key and a list of arguments corresponding to a list of localised formatted strings stored in Localizable.strings in the iPhone app.
function __push_data($push_notifications, $push_content) {
$ctx = stream_context_create();
if (ENVIRONMENT == 'development') {
stream_context_set_option($ctx, 'ssl', 'local_cert', 'dev.combined.pem');
} else if (ENVIRONMENT == 'production') {
stream_context_set_option($ctx, 'ssl', 'local_cert', 'prod.combined.pem');
}
stream_context_set_option($ctx, 'ssl', 'passphrase', A_PRIVATE_KEY);
/* Open a connection to the APNS server */
$apns_server = "ssl://gateway.sandbox.push.apple.com:2195";
if (ENVIRONMENT == 'production') {
$apns_server = "ssl://gateway.push.apple.com:2195";
}
$fp = stream_socket_client(
$apns_server, $err,
$errstr, 60, STREAM_CLIENT_CONNECT|STREAM_CLIENT_PERSISTENT, $ctx);
if ($fp) {
foreach ($push_notifications as $index => $push_notification) {
$body = array();
/* Create the payload body */
$body['aps'] = array('alert' => array('loc-key' => $push_notification['key'], 'loc-args' => $push_notification['message']), 'sound' => 'default');
$body['sopnc'] = $push_content;
/* Encode the payload as JSON */
$payload = json_encode($body);
/* Build the binary notification */
$msg = chr(0) . pack('n', 32) . pack('H*', $push_notification['push_token']) . pack('n', strlen($payload)) . $payload;
/* Send it to the server */
$result = fwrite($fp, $msg, strlen($msg));
if (!$result) {
echo 'Message not delivered' . PHP_EOL;
} else {
echo 'Message successfully delivered' . PHP_EOL;
}
}
/* Close the connection to the server */
fclose($fp);
}
}
Has somebody else encountered such problem?