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You must set a threshold for your Face Recognition Algorithm.For Example:

Ptr<FaceRecognizer> model = createEigenFaceRecognizer()
    model->set("threshold", 3000.0);

if the distance of your tested face from the trained face is above 3000.0 (threshold) then will be considered as unknown (-1).

See details here: http://docs.opencv.org/modules/contrib/doc/facerec/facerec_api.html