title: NodeJS Async: Callback already called?
body: <p>I'm using Async module in Node.JS to keep track of my asynchronous calls. However, I'm getting an error - "Callback already called." 
  Can someone help me out here? </p>
  
  <pre><code>async.each(data['results'], function(result, done) { 
      if (result['twitter_id'] !== null) { //Isolate twitter handle
          var param = { "user.screen_name": result['twitter_id']}
          db.test4.find( param, function(err, users) {
              if( err ) {
                  return done(err);
              } else if (!users) {
                  res.send("No user found");
              } else {
                  users.forEach( function(Result) { 
                      twitter_ids.push(Result);
                      //console.log(Result);
                      done();
                  });
              }
          });
      }
  }, function(err) {  
      if (err) {
          throw err
      }
      res.send(twitter_ids);
  });
  </code></pre>