Better enforce uniqueness for email delivery

Summary:
@skrul reports receiving multiple copies of notification emails since
@hunterbridges configured some bizarre dystopian email replication factory on
their outbound route. Two fixes:

  - Ensure "To" and "Cc" are unique. Email shouldn't be replicated for "To:
x@y.com, x@y.com" but it's silly that we do this.
  - Remove "To" addresses from "Cc". Email shouldn't be replicated here either,
but we don't really lose anything by accommodating this.

Test Plan:
Sent a mail to the same to/cc, verified I was to'd only and not cc'd when the
mail was delivered.

@hunterbridges, can you apply this patch locally and verify it fixes the issue?
You can test by going to MetaMTA -> Send New Message and sending a message to
yourself as both To and CC.

Reviewed By: skrul
Reviewers: skrul, hunterbridges, jungejason, tuomaspelkonen, aran
CC: aran, skrul, epriestley
Differential Revision: 751

src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php

@@ -88,11 +88,13 @@ class PhabricatorMetaMTAMail extends PhabricatorMetaMTADAO {
   }
 
   public function addTos(array $phids) {
+    $phids = array_unique($phids);
     $this->setParam('to', $phids);
     return $this;
   }
 
   public function addCCs(array $phids) {
+    $phids = array_unique($phids);
     $this->setParam('cc', $phids);
     return $this;
   }