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
