R1:6deddbfc701d

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…