Much asked-for change. Instead of throwing an early error when adding…

… duplicate models, Backbone will simply skip them instead.

backbone.js

@@ -501,7 +501,7 @@
     // Add a model, or list of models to the set. Pass **silent** to avoid
     // firing the `add` event for every new model.
     add: function(models, options) {
-      var i, index, length, model, cid, id, cids = {}, ids = {};
+      var i, index, length, model, cid, id, cids = {}, ids = {}, dups = [];
       options || (options = {});
       models = _.isArray(models) ? models.slice() : [models];
 
@@ -513,14 +513,21 @@
         }
         if (cids[cid = model.cid] || this._byCid[cid] ||
           (((id = model.id) != null) && (ids[id] || this._byId[id]))) {
-          throw new Error("Can't add the same model to a collection twice");
+          dups.push(i);
+          continue;
         }
         cids[cid] = ids[id] = model;
       }
 
+      // Remove duplicates.
+      i = dups.length;
+      while (i--) {
+        models.splice(dups[i], 1);
+      }
+
       // Listen to added models' events, and index models for lookup by
       // `id` and by `cid`.
-      for (i = 0; i < length; i++) {
+      for (i = 0, length = models.length; i < length; i++) {
         (model = models[i]).on('all', this._onModelEvent, this);
         this._byCid[model.cid] = model;
         if (model.id != null) this._byId[model.id] = model;
@@ -705,6 +712,7 @@
 
     // Prepare a model or hash of attributes to be added to this collection.
     _prepareModel: function(model, options) {
+      options || (options = {});
       if (!(model instanceof Backbone.Model)) {
         var attrs = model;
         options.collection = this;

test/collection.js

@@ -124,22 +124,15 @@ $(document).ready(function() {
   });
 
   test("Collection: can't add model to collection twice", function() {
-    raises(function(){
-      // no id, same cid
-      var a2 = new Backbone.Model({label: a.label});
-      a2.cid = a.cid;
-      col.push(a2);
-      ok(false, "duplicate; expected add to fail");
-    }, "Can't add the same model to a collection twice");
+    var col = new Backbone.Collection([{id: 1}, {id: 2}, {id: 1}, {id: 2}, {id: 3}]);
+    equal(col.pluck('id').join(' '), '1 2 3');
   });
 
   test("Collection: can't add different model with same id to collection twice", function() {
-    raises(function(){
-      var col = new Backbone.Collection;
-      col.unshift({id: 101});
-      col.add({id: 101});
-      ok(false, "duplicate; expected add to fail");
-    }, "Can't add the same model to a collection twice");
+    var col = new Backbone.Collection;
+    col.unshift({id: 101});
+    col.add({id: 101});
+    equal(col.length, 1);
   });
 
   test("Collection: add model to multiple collections", function() {
@@ -518,8 +511,11 @@ $(document).ready(function() {
   test("Collection: multiple copies of the same model", function() {
     var col = new Backbone.Collection();
     var model = new Backbone.Model();
-    raises(function() { col.add([model, model]); });
-    raises(function() { col.add([{id: 1}, {id: 1}]); });
+    col.add([model, model]);
+    equal(col.length, 1);
+    col.add([{id: 1}, {id: 1}]);
+    equal(col.length, 2);
+    equal(col.last().id, 1);
   });
 
   test("#964 - collection.get return in consistent", function() {