"Error: MongoError: E11000 duplicate key error collection: myFirstDatabase.users index: userName_1 dup key: { userName: null }"

[ShivashankarL]

While testing it in insomnia, got this kind of error

"Error: MongoError: E11000 duplicate key error collection: myFirstDatabase.users index: userName_1 dup key: { userName: null }"
can't insert more than one document