Code newbie here!

I'm following this guide to set up a backend server using Node.js, Express and MongoDB - https://www.codementor.io/@olatundegaruba/nodejs-restful-apis-in-10-minutes-q0sgsfhbd

I keep receiving the following error in the terminal when using Postman.

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

I looked into the fact I may need to add a return statement somewhere, but since I've been following the guide, I shouldn't be having this issue?

Here's my controllers file where I think the problem may be:

'use strict';

var mongoose = require('mongoose'),
  Task = mongoose.model('Tasks');

exports.listAllTasks = function (req, res) {
  Task.find({}, function (err, task) {
    if (err) res.send(err);
    res.json(task);
  });
};

exports.createATask = function (req, res) {
  var newTask = new Task(req.body);
  newTask.save(function (err, task) {
    if (err);
    res.send(err);
    res.json(task);
  });
};

exports.readATask = function (req, res) {
  Task.findById(req.params.taskId, function (err, task) {
    if (err) res.send(err);
    res.json(task);
  });
};

exports.updateATask = function (req, res) {
  Task.findOneAndUpdate(
    { _id: req.params.taskId },
    req.body,
    { new: true },
    function (err, task) {
      if (err) res.send(err);
      res.json(task);
    }
  );
};

exports.deleteATask = function (req, res) {
  Task.deleteMany(
    {
      _id: req.params.taskId,
    },
    function (err, task) {
      if (err) res.send(err);
      res.json({ message: 'Task successfully deleted' });
    }
  );
};

Thanks in advance!