zwrite should dedup CC lists

[andersk-auto]

See also, [https://barnowl.mit.edu/ticket/264 Barnowl #264]

[andersk-auto]

Imported from trac issue 121. Created by achernya@ATHENA.MIT.EDU on 2013-08-11T01:02:28, last modified: 2013-08-11T01:04:14