What makes a portrait unique? Your point in subject, your stance. How much do you know your subject, what do you feel about it? Do you have any personal relationships. Are you puzzled or are you indifferent?
You may not personally know your subject, but you need to know something about it. You need a piece of information so that you can attach yourself to it. Check out the portraits of all the great artists including Ara Güler or Steve McCurry. All of their frames have a unique style, an attachement, a signature. Their view of world, their understanding meets the reality of their subject; they melt in together. Those portraits are carefully selected and crafted.
As mentioned before, you don't need to personally know your subject, but you need to know something about it just to be able to make the decision whether to keep this frame or the other one. You need to feel which one of your shots represents your subject the best; and for that you need to know your subject; at least, you need to attach yourself to subject and try to see the world from its point of view.