Get the duplicate values of a field in MongoDB?

Use aggregate() method to get the duplicate value of a field. Let us first create a collection with documents using the following query

> db.findAllNonDistinctDemo.insertOne({"UserName":"John","UserAge":28});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c995078863d6ffd454bb647")
}
> db.findAllNonDistinctDemo.insertOne({"UserName":"Larry","UserAge":21});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c995081863d6ffd454bb648")
}
> db.findAllNonDistinctDemo.insertOne({"UserName":"Larry","UserAge":23});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c995089863d6ffd454bb649")
}
> db.findAllNonDistinctDemo.insertOne({"UserName":"David","UserAge":22});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c995093863d6ffd454bb64a")
}
> db.findAllNonDistinctDemo.insertOne({"UserName":"John","UserAge":26});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c99509d863d6ffd454bb64b")
}
> db.findAllNonDistinctDemo.insertOne({"UserName":"Robert","UserAge":24});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c9950a7863d6ffd454bb64c")
}
> db.findAllNonDistinctDemo.insertOne({"UserName":"Robert","UserAge":25});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c9950b1863d6ffd454bb64d")
}
> db.findAllNonDistinctDemo.insertOne({"UserName":"Mike","UserAge":29});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c9950bc863d6ffd454bb64e")
}

Following is the query to display all documents from a collection with the help of find() method

> db.findAllNonDistinctDemo.find().pretty();

This will produce the following output

{
   "_id" : ObjectId("5c995078863d6ffd454bb647"),
   "UserName" : "John",
   "UserAge" : 28
}
{
   "_id" : ObjectId("5c995081863d6ffd454bb648"),
   "UserName" : "Larry",
   "UserAge" : 21
}
{
   "_id" : ObjectId("5c995089863d6ffd454bb649"),
   "UserName" : "Larry",
   "UserAge" : 23
}
{
   "_id" : ObjectId("5c995093863d6ffd454bb64a"),
   "UserName" : "David",
   "UserAge" : 22
}
{
   "_id" : ObjectId("5c99509d863d6ffd454bb64b"),
   "UserName" : "John",
   "UserAge" : 26
}
{
   "_id" : ObjectId("5c9950a7863d6ffd454bb64c"),
   "UserName" : "Robert",
   "UserAge" : 24
}
{
   "_id" : ObjectId("5c9950b1863d6ffd454bb64d"),
   "UserName" : "Robert",
   "UserAge" : 25
}
{
   "_id" : ObjectId("5c9950bc863d6ffd454bb64e"),
   "UserName" : "Mike",
   "UserAge" : 29
}

Following is the query to find all the duplicate values of a field in MongoDB

> db.findAllNonDistinctDemo.aggregate([
...    { "$group": {
...       "_id": "$UserName",
...       "Counter": { "$sum": 1 }
...    }},
...    { "$match": {
...       "Counter": { "$gt": 1 }
...    }}
... ]).pretty();

This will produce the following output. All the duplicate records are visible now

{ "_id" : "Robert", "Counter" : 2 }
{ "_id" : "Larry", "Counter" : 2 }
{ "_id" : "John", "Counter" : 2 }