我尝试使用 mongodump（版本 3.2.5）备份 MongoDB（版本 2.4.9）。它成功了。但我无法恢复此备份。为什么？
./mongorestore -h 127.0.0.1 -u xxx -p xxx --dir /home/jonkyon/mongo_2 --authenticationDatabase admin --drop
2016-04-25T19:08:24.028+0800 building a list of dbs and collections to restore from /home/jonkyon/mongo_2 dir
2016-04-25T19:08:24.029+0800 assuming users in the dump directory are from <= 2.4 (auth version 1)
2016-04-25T19:08:24.030+0800 cannot drop system collection products.system.users, skipping
2016-04-25T19:08:24.031+0800 reading metadata for products.system.users from /home/jonkyon/mongo_2/products/system.users.metadata.json
2016-04-25T19:08:24.031+0800 restoring products.system.users from /home/jonkyon/mongo_2/products/system.users.bson
2016-04-25T19:08:24.032+0800 error: E11000 duplicate key error index: products.system.users.$_id_ dup key: { : ObjectId('570e2f0ca19b9c2cb7e75905') }
2016-04-25T19:08:24.066+0800 restoring indexes for collection products.system.users from metadata
2016-04-25T19:08:24.068+0800 reading metadata for runoob.runoob from /home/jonkyon/mongo_2/runoob/runoob.metadata.json
2016-04-25T19:08:24.070+0800 finished restoring products.system.users (2 documents)
2016-04-25T19:08:24.070+0800 restoring runoob.runoob from /home/jonkyon/mongo_2/runoob/runoob.bson
2016-04-25T19:08:24.070+0800 restoring indexes for collection runoob.runoob from metadata
2016-04-25T19:08:24.071+0800 finished restoring runoob.runoob (2 documents)
2016-04-25T19:08:24.071+0800 restoring users from /home/jonkyon/mongo_2/admin/system.users.bson
2016-04-25T19:08:24.088+0800 Failed: restore error: error running merge command: no such cmd: _mergeAuthzCollections