Duplicate key error in collection

[adriaanmeuris]

I've set up the module and seems to be working fine, however I'm getting duplicate key errors:
errmsg: 'E11000 duplicate key error collection: mydb.ratelimits index: _id_ dup key: { : "G/IAnx1Agvj9X0JzMvChzHFl6jKU0ps4dnfVvJ+ZYTc=" }' } }

I've ensured the index to be set, this is my implementation:

var MongoStore = require('express-brute-mongo');
var MongoClient = require('mongodb').MongoClient;

var bruteStore = new MongoStore(function (ready) {
  MongoClient.connect(process.env['DB_URL'], function (err, db) {
    if (err) throw err;
    db.collection('ratelimits').ensureIndex({expires: 1}, {expireAfterSeconds: 0});
    ready(db.collection('ratelimits'));
  });
});

This is the middleware I set up (same as defaults, but logging the dup key error):

var rateLimitMiddleware = new ExpressBrute(bruteStore, {
  failCallback: function (req, res, next, nextValidRequestDate) {
    var err = new Error('Too many attempts.');
    err.status = 429;
    next(err);
  },
  handleStoreError: function (error) {
    console.log(error);
  }
});

Any ideas?

[adriaanmeuris]

FYI, this happens when the module tries to increment the request count (error message: "Cannot increment request count")