If you remove the callback parameter we can simplify this test to::

```javascript
it('verify cannot register non-unique search document producers', function() {
    SearchAPI.registerSearchDocumentProducer('test-registerSearchDocumentProducer', function() { });

    // Try and register a second document producer of the same type and assert the API throws an error
    assert.throws(function() {
        SearchAPI.registerSearchDocumentProducer('test-registerSearchDocumentProducer', function() { });
    }, Error);
});
```
[1] http://nodejs.org/api/assert.html#assert_assert_throws_block_error_message