title: Callback was already called async parallel
body: <p>Hi I am trying to use the Async module to retrieve two users and do some processing after they have both been retrieved however I keep getting the error message: Callback was already called. Below is the code i currently have:</p>
  
  <pre><code>app.get('/api/addfriend/:id/:email', function(req, res) {
      var id = req.params.id;
      var friendEmail = req.params.email;
      async.parallel([
              //get account
              function(callback) {
                  accountsDB.find({
                      '_id': ObjectId(id)
                  }, function(err, account) {
                      console.log(id);
                      if (err || account.length === 0) {
                          callback(err);
                      }
                      console.log(account[0]);
                      callback(null, account[0]);
                  });
              },
              //get friend
              function(callback) {
                  accountsDB.find({
                      'email': friendEmail
                  }, function(err, friend) {
                      console.log(friendEmail);
                      if (err || friend.length === 0 || friend[0].resId === undefined) {
                          callback(err);
                      }
                      console.log(friend[0]);
                      callback(null, friend[0].resId);
                  });
              }
          ],
  
          //Compute all results
          function(err, results) {
              if (err) {
                  console.log(err);
                  return res.send(400);
              }
  
              if (results === null || results[0] === null || results[1] === null) {
                  return res.send(400);
              }
  
              //results contains [sheets, Friends, Expenses]
              var account = results[0];
              var friend = results[1];
              if (account.friends_list !== undefined) {
                  account.friends_list = account.friends_list + ',' + friend;
              }
              else {
                  account.friends_list = friend;
              }
              // sheetData.friends = results[1];
              accountsDB.save(
                  account,
                  function(err, saved) {
                      if (err || !saved) {
                          console.log("Record not saved");
                      }
                      else {
                          console.log("Record saved");
                          return res.send(200, "friend added");
                      }
                  }
              );
  
          }
      );
  });
  </code></pre>
  
  <p>Any help would be appreciated.</p>