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