Kentico claims described behavior to be a feature:
"Duplicate recipient email addresses
Even though all regular marketing email recipients need to have a unique email address, it is still possible to have a single address subscribed to a marketing email multiple times through contact groups. In these cases, the system filters out the duplicate addresses when sending the emails. This way each email address only receives the marketing email once."
More details here.
Unfortunately I couldn't find any event/place to hook into in order to override that behavior.
I've tried to locate Provider that actually gets all the recipient from the database and filters out duplicates, but couldn't find anything specific in documentation. I believe it is worth asking Kentico support - I believe they should be able to tell you what class and method you need to override.Keep in mind once you customize this logic Kentico wont longer track duplicates, so I'd recommend to double check recipients lists for all the campaigns and newsletters.