Fixes #836, Fixes #708 -- going back to previous stance: two models w…

…ith the same id can't be added to the same collection.

backbone.js

@@ -422,10 +422,12 @@
       models = slice.call(models);
       for (i = 0, l = models.length; i < l; i++) {
         var model = models[i] = this._prepareModel(models[i], options);
-        if (this._byCid[model.cid]) {
+        var hasId = model.id != null;
+        if (this._byCid[model.cid] || (hasId && this._byId[model.id])) {
           throw new Error("Can't add the same model to a set twice");
         }
-        this._byId[model.id] = this._byCid[model.cid] = model;
+        this._byCid[model.cid] = model;
+        if (hasId) this._byId[model.id] = model;
         model.bind('all', this._onModelEvent, this);
         this.length++;
       }

test/collection.js

@@ -136,7 +136,7 @@ $(document).ready(function() {
     }
   });
 
-  test("Collection: add model to collection twice", function() {
+  test("Collection: can't add model to collection twice", function() {
     try {
       // no id, same cid
       var a2 = new Backbone.Model({label: a.label});
@@ -148,6 +148,17 @@ $(document).ready(function() {
     }
   });
 
+  test("Collection: can't add different model with same id to collection twice", function() {
+    var col = new Backbone.Collection;
+    try {
+      col.add({id: 101});
+      col.add({id: 101});
+      ok(false, "duplicate; expected add to fail");
+    } catch (e) {
+      equals(e.message, "Can't add the same model to a set twice");
+    }
+  });
+
   test("Collection: add model to multiple collections", function() {
     var counter = 0;
     var e = new Backbone.Model({id: 10, label : 'e'});