node-mongodb-native について(エラーがでるコード)

gistfile1.txt

> var server = new (require('mongodb').Server)('localhost', 27017);
> var db = new (require('mongodb').Db)('foo', server);
> db.open(function(){
...   db.collection('foo', function(_, collection){
...      collection.insert([{"_id": 'foo'}, {"_id": 'foo'}], {safe:true}, function(e){ console.log(e); })
...   });
... });
> { stack: [Getter/Setter],
  arguments: undefined,
  type: undefined,
  message: 'E11000 duplicate key error index: foo.foo.$_id_  dup key: { : "foo" }',
  name: 'MongoError',
  err: 'E11000 duplicate key error index: foo.foo.$_id_  dup key: { : "foo" }',
  code: 11000,
  n: 0,
  connectionId: 1,
  ok: 1 }