See http://blog.shinetech.com/2012/12/24/an-identity-map-for-backbone-js/

backbone-identity.coffee

setPrototype = Object.setPrototypeOf or (child, parent) -> child.__proto__ = parent
inherit = (parent, child) -> setPrototype child, parent; child.prototype=parent.prototype; child
 
IdentityMap = (realConstructor) ->
  cache = {}
  id = realConstructor::idAttribute
  inherit realConstructor, (attributes, options) ->
    create = -> new realConstructor attributes, options
 
    if objectId = attributes?[id]
      if (object = cache[objectId])?
        object.set (options?.parse? attributes) or attributes
      else cache[objectId] = create()
    else
      create().once 'change:' + id, (model, objectId) -> cache[objectId] = model


# Usage
SomeClass = IdentityMap class extends Backbone.Model
  validate: -> # ...
  # ...