Error event listener called twice

I'm simply subscribing to `error` events as said in docs, but my handler gets called twice. Why is it so and how to avoid that?

``` javascript
let kue = require('kue');
let queue = kue.createQueue();

queue.on('error', e => console.log('Got error', e.message));
```

Start with the documented `queue.on('error', ...)` example and trace the error-event path beginning at `createQueue()`. Confirm why the handler fires twice and define done as a documented, reproducible way to avoid duplicate notifications.

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