Question
The same e-mail address was assigned multiple Lead Ids -- why, and how to avoid it?
I'm also asking our friendly integration consultant, but given that I'm working late, I thought I'd try my luck here, too.

How come the same mail address is assigned two different LeadIds ?
I was under the impression that the email address is a primary look-up key for leads and should not be mapped to more than one lead.
These leads are created using the create-or-update-leads JSON API with no custom merge/lookup key.