Implementing delivery using SMTP:  Duplications of the same message will appear in the archive is emails are sent to more than one person, this is because of the design of the routing rules to send a copy of each message received for each recipient.  BCC and CC recipients are maintained.

Implementing using POP or delivery to an additional Gmail mailbox first:  Messages are combined as a single message for multiple recipients, however, BCC recipients are not identified.
That is by design of Google.