That's the correct place, yes.

is it OK?
  turba/lib/Api.php:682

        // Check if the entry already exists in the data source.
$unique_key = 'email';
if (array_key_exists($unique_key, $content)) {
         $result = $driver->search(array($unique_key => 
$content[$unique_key]));
} else {
         $result = $driver->search($content);
}

Please help me modify the code locally.
I got this far:
imp/lib/Compose.php:1187
$registry->call('contacts/import', array(array('name' => $name, 
'email' => $recipient->bare_address), 'array', $abook));

Where do I find the registered 'contacts/import' funcition?
Thank you!

Could you please revert the functionality (as was in H4) to match ONLY 
the email address?

That's the expected behavior. It searches all information available, 
i.e. name and address in this case. Only if all fields match, a 
contact is considered a duplicate.

YES, I am getting duplicates. If there is no entry with exactly the 
same name, turba creates another entry with the same address. I 
suspect turba compares the names, not the address.
e.g.
Ételmanufaktúra, Mannatural,        info@mannatural.hu
Ételmanufaktúra,, Mannatural,,        info@mannatural.hu
Mannatural, Ételmanufaktúra        info@mannatural.hu
Mannatural,, Ételmanufaktúra,        info@mannatural.hu

This exactly the opposite from what you've written in the summary. So 
*are* you getting duplicates or *not*? And if you do, under which 
circumstances?

"Save recipients automatically to the default address book?" is on.
When I write a message to someone his/her name get into the address book
IF the there is a difference between the new name and the name already 
in the address book. Same names don't get duplicated.

Thank you!