Example shell output:

> db.dup.insert({_id:1, a:1, a:2})
WriteResult({ "nInserted" : 1 })
> db.dup.find()

{ "_id" : 1, "a" : 2 }

InsertMany has the same problem:

> db.dup.insertMany([\{_id:1, a:1, a:2},\{_id:2,a:1,a:2}])

{ "acknowledged" : true, "insertedIds" : [ 1, 2 ] }

> db.dup.find()

{ "_id" : 1, "a" : 2 } { "_id" : 2, "a" : 2 }