Throw error if there is an attempt to associate multiple handlers to the same queeu

```
workers: [
    {
      queue: 'q1',
      name: 'workerA',
      concurrency: 1,
      handler: handler1
    },
    {
      queue: 'q1',
      name: 'workerB',
      concurrency: 1,
      handler: handler1b
    }
  ]
```