Overwrite issue in merge_vars of Mandrill API

I am sending email using merge_vars for dynamic contents. Here is how my merge_vars look like:

$message['merge_vars'][$index] = array(
  'rcpt' => $email,
  'vars' => array(
    array(
      'name' => 'url',
      'content' => $url
    ),
    array(
      'name' => 'sname',
      'content' => $sname
    ),
    array(
      'name' => 'lname',
      'content' => $lname,
    ),
    array(
      'name' => 'email',
      'content' => $email
    )
  ),
);

Everything is working fine. But when same recipient should receive multiple different email in a single API call, then the problem occurs. That time same recipient don't receive different email, he receives same email multiple times.