I can't repro with a gmail acct. The bcc is visible in both outlook and in the account online... and the wonderful gmail bug that resends messages outlook sent (when you save sent messages to the gmail folder) is alive and well.
Oh... i reproed it. The fix for the duplicate gmail message appears to be the cause. File, Account settings, double click on the gmail account - then More sEttings, Sent folder tab. what is your save sent items setting?
The choices are to save sent items, save locally, or no save. If you save sent items to the gmail sent folder (looks like its the default setting) gmail resends the message. If you choose not to save, gmail saves a copy of the sent message but does not include the bcc data in the sent copy. It should work (and not send duplicates) if you save the sent items locally, but i didn't test it.