Duplicate value inserted in the mongodb "_id" field - php

Question

In mongodb, We can assign our own value to _id field and the "_id" field value may be of any type, other than arrays, so long as it is a unique -- From the docs.
But in my live database, i can see some records were duplicated as follows,
db.memberrecords.find().limit(2).forEach(printjson)
{
"_id" : "999783",
"Memberid" : "999783",
"Name" : "ASHEESH SHARMA",
"Gender" : "M",
}
{
"_id" : "999783",
"Memberid" : "999783",
"Name" : "Sanwal Singh Meena",
"Gender" : "M",
}
In above records, the same _id value inserted twice in the table. When i tested with local database it is not allowing to insert the same _id record and throwing error as follows,
E11000 duplicate key error index: mongoms.memberrecords.$_id_ dup key: { : "999783" }
Below is the Indexes for my live memberrecords table(for your reference),
db.memberrecords.getIndexes()
[
{
"name" : "_id_",
"ns" : "mongoms.memberrecords",
"key" : {
"_id" : 1
},
"v" : 0
},
{
"_id" : ObjectId("4f0bcdf2b1513267f4ac227c"),
"ns" : "mongoms.memberrecords",
"key" : {
"Memberid" : 1
},
"name" : "Memberid_1",
"unique" : true,
"v" : 0
}
]
Note: i have two sharding for this table.
Any suggestion on this please,

Answer 1

Is your shard key the _id field? You can only have one unique index enforced across a cluster: the shard key (otherwise the server would have to check with every shard on every insert).
So: on a single a shard, _id will be unique. However, if it isn't your shard key, all bets are off across multiple shards.
See http://www.mongodb.org/display/DOCS/Sharding+Limits#ShardingLimits-UniqueIndexesDOCS%3AIndexes%23UniqueIndexes.