When adding participants to a survey there is no check if the added participants already exist.
In line 1878, inside the foreach loop of the participants that should be added, the new token is created without any previous check if he/she already exists.
I solved adding the following code before line 1878

            $model =Token::model($iSurveyID)->findByAttributes(array(
                'firstname' => $aParticipant['firstname'],
                'lastname'  => $aParticipant['lastname'],
                'email'     => $aParticipant['email']
            ));
            if ($model) {
                $aParticipant["errors"] = "Participant already existent";
                continue;
            }

Anyway this is just one possible solution. Another one would be allowing the update of already existing participants. Also the check if the participant already exists could be done on just the email field rather than the three fields I used.