R1:570a78d42af7

don't add email addresses to CC if they are already in TO

Summary:
Some mailers remove the duplicate entries themselves, but some (Mailgun) don't.
This affects installations with metamta.one-mail-per-recipient set to false, and will cause
- ugly looking "to" entries. Gmail, for example, collapses to+cc entries to one list, so you get something that looks like "to: me me john"
- It sometimes causes duplicate delivery of the same message when used in conjuction with Google Groups. I suspect that their…