Unique indexes ensure that a value appears at most once for a given field.
To create a unique index in the MongoDB Shell, use the
db.collection.createIndex() method with the unique option
set to true.
db.collection.createIndex( { <field>: <sortOrder> }, { unique: true } )
About this Task
This example adds a unique index on the user_id field of a
members collection to ensure that there are no duplicate values in
the user_id field.
Steps
To create a unique index on the user_id field of the members
collection, run the following command in mongosh:
db.members.createIndex( { "user_id": 1 }, { unique: true } )