How to remove duplicates from MongoDB Collection?

For this, set “unique:true” i.e. the unique constraint and avoid inserting duplicates as in the below syntax −

db.yourCollectionName.ensureIndex({yourFieldName: 1}, {unique: true, dropDups: true})

To understand the above syntax, let us create a collection with documents. Here, duplicate insertion won’t be allowed −

> db.demo604.ensureIndex({FirstName: 1}, {unique: true, dropDups: true});{    "createdCollectionAutomatically" : true,    "numIndexesBefore" : 1,    "numIndexesAfter" : 2,    "ok" : 1 } > db.demo604.insertOne({FirstName:"Chris"});{    "acknowledged" : true,    "insertedId" : ObjectId("5e960887ed011c280a0905d8") } > db.demo604.insertOne({FirstName:"Bob"});{    "acknowledged" : true,    "insertedId" : ObjectId("5e96088aed011c280a0905d9") } > db.demo604.insertOne({FirstName:"David"});{    "acknowledged" : true,    "insertedId" : ObjectId("5e96088ded011c280a0905da") } > db.demo604.insertOne({FirstName:"Chris"}); 2020-04-15T00:31:35.978+0530 E QUERY [js] WriteError: E11000 duplicate key error collection: test.demo604 index: FirstName_1 dup key: { : "Chris" } : WriteError({    "index" : 0,    "code" : 11000,    "errmsg" : "E11000 duplicate key error collection: test.demo604 index: FirstName_1 dup key: { : \"Chris\" }",    "op" : {       "_id" : ObjectId("5e96088fed011c280a0905db"),       "FirstName" : "Chris"    } }) WriteError@src/mongo/shell/bulk_api.js:461:48 Bulk/mergeBatchResults@src/mongo/shell/bulk_api.js:841:49 Bulk/executeBatch@src/mongo/shell/bulk_api.js:906:13 Bulk/this.execute@src/mongo/shell/bulk_api.js:1150:21 DBCollection.prototype.insertOne@src/mongo/shell/crud_api.js:252:9 @(shell):1:1

Display all documents from a collection with the help of find() method −

> db.demo604.find();

This will produce the following output −

{ "_id" : ObjectId("5e960887ed011c280a0905d8"), "FirstName" : "Chris" } { "_id" : ObjectId("5e96088aed011c280a0905d9"), "FirstName" : "Bob" } { "_id" : ObjectId("5e96088ded011c280a0905da"), "FirstName" : "David" }