This is actually not difficult.. If you know of another user sharing your connection, e-mail multi-accounts with the handles so you can register as multis. If you do not know of another user sharing your connection.. obviously there is nothing you can e-mail them, so that's that. There are only 4,000 players here across the globe, and the world population is currently closing in on 7,000,000,000 -- so obviously the odds are slim at best that someone will just happen to be a member and log on from your connection without your knowledge.
Yes, we can tell the difference between public IPs and proxies, as opposed to private IPs, and no, people do not normally get mistakenly banned. If you do, simply mail multi-accounts to explain your situation and it will be resolved. It is very easy to confirm your story.