Duplication Steps

• Create a model, which parses it's id from an arbitrary response object attribute
• Call fetch on a collection for the model
  • API returns multiple models with the same id
• Call fetch a second time
  • API returns multiple models with the same id
• Backbone throws error at 719:46 TypeError: Cannot read property 'cid' of undefined

set: function(models, options) {
      // ....

      // Remove nonexistent models if appropriate.
      if (remove) {
        for (i = 0, l = this.length; i < l; ++i) {
          if (!modelMap[(model = this.models[i]).cid]) toRemove.push(model);
          // TypeError; this.models.length !== this.length
        }
        if (toRemove.length) this.remove(toRemove, options);
      }

      // ...
}

See Spec (failing) for mismatched collection length for a failing spec, duplicating this issue.

Expected Behavior

The root issue is clearly that the API is returning duplicate objects. However, I'm wondering if there might be a better way for Backbone to handle this situation, which would make the problem easier to trace.

What is the default behavior when adding a model to a collection with the same id as an existing model in the collection? I would expect an error to be thrown.