de-duplicate emails received by phabricator multiple times

Summary: this can happen if you have Phabricator and email lists co-mingling such that Phabricator receives an email multiple times. we can prevent this from then spamming everyone or otherwise taking the action multiple times by storing a message id hash and dropping the message if we have more than one message that matches.

Test Plan: simulated sending the same email multiple times on the command line. noted only the first one made it through.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1726

Differential Revision: https://secure.phabricator.com/D4328

scripts/mail/mail_handler.php

@@ -34,6 +34,9 @@ $received->setBodies(array(
   'text' => $text_body,
   'html' => $parser->getMessageBody('html'),
 ));
+$received->setMessageIDHash(
+  PhabricatorHash::digestForIndex($received->getMessageID())
+);
 
 $attachments = array();
 foreach ($parser->getAttachments() as $attachment) {