MongoDB query to group duplicate documents

To group duplicate documents in MongoDB, use the $group stage in the aggregation pipeline. This operation groups documents by a specified field and eliminates duplicates by creating a single document per unique value.

Syntax

db.collection.aggregate([
    { $group: { _id: "$fieldName" } }
]);

Sample Data

db.demo501.insertMany([
    { "Name": "Chris" },
    { "Name": "Bob" },
    { "Name": "Chris" },
    { "Name": "John" },
    { "Name": "Chris" },
    { "Name": "John" },
    { "Name": "David" }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e8752f0987b6e0e9d18f566"),
        ObjectId("5e8752f4987b6e0e9d18f567"),
        ObjectId("5e8752f8987b6e0e9d18f568"),
        ObjectId("5e8752fb987b6e0e9d18f569"),
        ObjectId("5e8752fd987b6e0e9d18f56a"),
        ObjectId("5e875301987b6e0e9d18f56b"),
        ObjectId("5e875307987b6e0e9d18f56c")
    ]
}

Display all documents to see the duplicates ?

db.demo501.find();

{ "_id": ObjectId("5e8752f0987b6e0e9d18f566"), "Name": "Chris" }
{ "_id": ObjectId("5e8752f4987b6e0e9d18f567"), "Name": "Bob" }
{ "_id": ObjectId("5e8752f8987b6e0e9d18f568"), "Name": "Chris" }
{ "_id": ObjectId("5e8752fb987b6e0e9d18f569"), "Name": "John" }
{ "_id": ObjectId("5e8752fd987b6e0e9d18f56a"), "Name": "Chris" }
{ "_id": ObjectId("5e875301987b6e0e9d18f56b"), "Name": "John" }
{ "_id": ObjectId("5e875307987b6e0e9d18f56c"), "Name": "David" }

Group Duplicate Documents

Use $group to get unique values from the Name field ?

db.demo501.aggregate([
    { $group: { _id: "$Name" } }
]);

{ "_id": "David" }
{ "_id": "John" }
{ "_id": "Bob" }
{ "_id": "Chris" }

Count Duplicates

Add a count field to see how many duplicates exist for each name ?

db.demo501.aggregate([
    { 
        $group: { 
            _id: "$Name", 
            count: { $sum: 1 } 
        } 
    }
]);

{ "_id": "David", "count": 1 }
{ "_id": "John", "count": 2 }
{ "_id": "Bob", "count": 1 }
{ "_id": "Chris", "count": 3 }

Conclusion

The $group stage effectively eliminates duplicates by grouping documents with the same field value. Use $sum: 1 to count occurrences of each unique value for better duplicate analysis.