I have a collection created with below index. Only name is of type string and check should be done in case insensitive manner. Only allows 'A' or 'a'

```
db.collection('myCollection').createIndex(
          { "name": 1, "type": 1, "ref": 1 }, { unique: true, collation: { locale: 'en', strength: 1 }});
      })
    }

```

I have tested and it works but I don't understand the error message:

```
{
    "errorMessage": "E11000 duplicate key error collection: my.myCollection index: name_1_type_1_ref_1 dup key: { : \"O1XO\u0006\u000c)MN11\", : \")MB1O1FG1\", : null }"
}

```

What does the dup key part mean? Is this the correct way to do this?