Handle case of duplicate messages when using Fetch (EAS 2.5)

* It turns out that it's possible to have two messages with the
  same server id and mailbox; this causes our HTML-loading code
  for EAS 2.5 to loop indefinitely.
* It's unclear whether the cause is ours or Hotmail's, but the
  fix simply deletes the duplicate when it's detected

Bug: 5789201

Change-Id: I42e7cd9a4b3b3693dae38ac93941bb69f31b0051
1 file changed