title: How to fix the callback was already called in loopback model
body: <p>I have tried to extend api in loopback model.
  In the model, I used standard api of model such as findOne, create.
  The example codes are below</p>
  
  <pre><code>Subscriber.findOne({
   where : {
        email : "............."
   },
   function(err, instance){
        if(instance)
        {
             cb(null,instance);
             response = "success";
        }
   }
   cb(null, response);
  </code></pre>
  
  <p>But when I called this extended api, the error was occurred.</p>
  
  <pre><code>throw err:// Rethron non-MsSQL errors
      ^
  
  Error: Callback was already called.
  </code></pre>
  
  <p>How to fix this error?         </p>