Trigger the fetchById callback with the model that actually got added to the collection.
Trigger the fetchById callback with the model that actually got added to the collection.

It's possible that a separate fetch completes while we are calling fetchById, in that case we want to ensure we only have a single reference to the model with a given id.
dhritzkiv/ampersand-collection-rest-mixin,AmpersandJS/ampersand-collection-rest-mixin