function print(str) {

  document.body.innerHTML += "<pre>" + str + "</pre>";

}

​

var Model = Backbone.Model.extend({

  defaults: {

    prop: null,

  }

});

var Collection = Backbone.Collection.extend({

  model: Model,

  _add: function(model, params) {   

    if (!params || !params.merge) {

        if (!this.where(model).length) this.add(model, params);

    } else {

        this.add(model, params);

    }

    return this;

  }

});

​

var collection = new Collection([{prop: "A"}, {prop: "B"}, {prop: "A"}]);

​

collection._add({ prop: "B" });

collection._add({ prop: "C" });

​

print(JSON.stringify(collection.toJSON()));

​

collection._add({prop: "B"}, {merge: true});

​

print(JSON.stringify(collection.toJSON()));