function print(str) {

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

}

​

var Model = Backbone.Model.extend({

  defaults: {

    prop: null,

  }

});

var Collection = Backbone.Collection.extend({

  model: Model

});

​

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

​

collection.on("add", function(model, collection, params) {

    if (!params.merge) {

        if (collection.where({ prop: model.get("prop") }).length > 1) {

            collection.remove(model);

        }

    }   

});

​

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

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

​

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

​

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

​

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