new backbone.js

// If a duplicate is found, prevent it from being added and
// optionally merge it into the existing model.
existing = this.get(attrs);
if (!existing) {
  model = newModels[i] = this._prepareModel(attrs, options);
  if (!model) continue;
  existing = this.get(model);
}
if (existing) {
  ....
} else if (add) {
  toAdd.push(model);
  this._addReference(model, options);
}