Quite a minor bug. Version 7.1.30794.0 on Windows 10.
Menu > Tools > Accounts > General tab > Add bcc address.
Start a new email. BCC address is correctly added in the BCC field.
Type a subject. Close the email and keep the draft.
Open the drafts folder and double click the email you saved. The BCC address is duplicated.
Repeat by making an amendment, saving and re-opening. The BCC address is duplicated once for each time the draft email is re-opened.