E11000 duplicate key error index: demo.cities.$country_code_1_name_1 dup key: { : "AR", : "Mercedes" }

Remove the offending key and try again: > db.cities.remove( { 'country_code': 'AR', name: 'Mercedes' } ); > db.cities.createIndex( { country_code: 1, name: 1 }, { unique: true } ); E11000 duplicate key error index: demo.cities.$country_code_1_name_1 dup key: { : "AR", : "San Pedro" }

You can force the index with *dropDups* (|ff9999|This could delete a lot of data!|): > db.cities.createIndex( { country_code: 1, name: 1 }, { unique: true, *|f10000|dropDups : true|* } );