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 }