The function cpd_importParticipants(integer $sSessionKey, array $aParticipants) : array
checks for duplicates before doing an insert.

It is checked with:

$arRecordExists = Participant::model()->exists(
'firstname = :firstname AND lastname = :lastname AND email = :email AND owner_uid = :owner_uid',
array(
':firstname' => $aData['firstname'],
':lastname' => $aData['lastname'],
':email' => $aData['email'],
':owner_uid' => $aData['owner_uid'],
));

If no firstname and lastname is specified and a praticipant only identified by email no participant is returned and you can insert same data again.
If a firstname and lastname is specified, everything works fine.