Insert in MongoDB without duplicates

To insert records in MongoDB and avoid duplicates, create a unique index on the field that should not contain duplicate values. When you attempt to insert a document with a duplicate value, MongoDB will reject the operation with an error.

Syntax

db.collection.createIndex({"fieldName": 1}, { unique: true });
db.collection.insert({"fieldName": "value"});

Example

Let's create a unique index on the StudentFirstName field and attempt to insert duplicate records ?

Step 1: Create Unique Index

db.insertWithoutDuplicateDemo.createIndex({"StudentFirstName":1},{ unique: true });

{
   "createdCollectionAutomatically" : true,
   "numIndexesBefore" : 1,
   "numIndexesAfter" : 2,
   "ok" : 1
}

Step 2: Insert Documents

db.insertWithoutDuplicateDemo.insertMany([
    {"StudentFirstName": "Chris"},
    {"StudentFirstName": "David"},
    {"StudentFirstName": "Bob"}
]);

{
   "acknowledged" : true,
   "insertedIds" : [
      ObjectId("5e064405150ee0e76c06a054"),
      ObjectId("5e064410150ee0e76c06a055"),
      ObjectId("5e06441f150ee0e76c06a057")
   ]
}

Step 3: Attempt Duplicate Insert

Now try inserting a duplicate "Chris" record ?

db.insertWithoutDuplicateDemo.insert({"StudentFirstName": "Chris"});

WriteError({
   "nInserted" : 0,
   "writeError" : {
      "code" : 11000,
      "errmsg" : "E11000 duplicate key error collection: test.insertWithoutDuplicateDemo index: StudentFirstName_1 dup key: { : "Chris" }"
   }
})

Verify Results

db.insertWithoutDuplicateDemo.find().pretty();

{
   "_id" : ObjectId("5e064405150ee0e76c06a054"),
   "StudentFirstName" : "Chris"
}
{
   "_id" : ObjectId("5e064410150ee0e76c06a055"),
   "StudentFirstName" : "David"
}
{
   "_id" : ObjectId("5e06441f150ee0e76c06a057"),
   "StudentFirstName" : "Bob"
}

Key Points

• The unique index prevents duplicate values in the specified field
• MongoDB returns error code 11000 when attempting to insert duplicates
• Existing documents remain unchanged when duplicate inserts fail

Conclusion

Creating a unique index is the most effective way to prevent duplicate records in MongoDB. The database automatically rejects any insert operation that would create duplicate values in the indexed field.