I personally see that most of the LN email users are keep TO/CC and BCC him/herself when sending the email to the others, which come into notice that duplication of data is now most of the user's habit. As per my findings still there is no restriction is available for TO / CC and BCC field. (Currently if you have some development experience then you can block it, but it need to be manageable through parameter or some other server level.
My advice to handle this through followings.
1, Server Policy
2, Client /Server Notes.ini parameter level
3, Environment variables on client notes.ini (when user start the LN then get the login User and restrict him/her for keep the login user name into TO / CC and BCC field. but this only work for LN Client user not work able for Web access users.