I just got off the phone with support (for an hour) and there's been a HUGE change that was not communicated to developers.
THE APIs NOW ACCEPT DUPLICATE EMAIL ADDRESSES.
When your account moves to the new contact format (I'm not sure what percentage of customers are already converted), the APIs will no longer filter out duplicates. If you make a request to add a contact whose address already exists, it will be added even if the email address is already in your contacts.
This also affects bulk imports via the APIs! Beware!
Needless to say, I'm very upset that this wasn't properly communicated to developers. This will take me weeks to fix in my applications.