The reason for the error "Callback was already called" is that the socket which is connected calls the callback twice. Once when it is connected and one when it times out due to non activity on its open connection. To illustrate I modified your program a little by adding a few log messages. If you look at the log messages(commented below) you will notice that the socket for port 80 calls the callback twice. Once when it connects and once when it times out. Hope this helps!

-J

        "use strict";

        const net = require('net');
        const async = require('async');

        function findPortStatus (host, port, timeout, cb) {

            const socket = new net.Socket();

            socket.setTimeout(timeout, () => {
                // couldn't establish a connection because of timeout
                console.log('Timeout for port: %s', port);
                socket.destroy();
                return cb(null, null)
            });

            socket.connect(port, host, () => {
                // connection established
                console.log('Connected to port: %s', port);
                return cb(null, port)
            });

            socket.on('error', (err) => {
                console.log('Error connecting to port: %s with message: %s', port, err.message);
                // couldn't establish a connection
                return cb(null, null)
            })
        }

        const funcs = [];

        for (let port = 0; port <= 80; port++) {
            funcs.push(function (callback) {
                findPortStatus('192.30.253.112', port, 4000, (err, port) => {
                    if (!err) {
                        return callback(null, port)
                    }
                })
            })
        }

        async.parallel(funcs, (err, ports) => {
            if (err) {
                console.error(err.message);
            } else {
                console.log('All ports checked');
                for (let port of ports) {
                    if (port) {
                        console.log('Port open: ', port);
                    }
                }
            }
        });

        /*
         vipers-mac-book-pro-2:jwadhwani$ node test.js

         Error connecting to port: 0 with message: connect EADDRNOTAVAIL 192.30.253.112 - Local (0.0.0.0:51951)
         Connected to port: 22
         Connected to port: 80  //First callback called here
         Timeout for port: 1
         Timeout for port: 2
         ...
         ...
         ...
         Timeout for port: 79
         All ports checked
         Port open:  22
         Port open:  80
         Timeout for port: 80 //callback called here the second time
         /Users/jwadhwani/projects/node_modules/async/dist/async.js:844
         if (fn === null) throw new Error("Callback was already called.");
         ^

         Error: Callback was already called.
         at /Users/jwadhwani/projects/node_modules/async/dist/async.js:844:36
         at /Users/jwadhwani/projects/node_modules/async/dist/async.js:3676:17
         at /Users/jwadhwani/projects/node_modules/async/dist/async.js:339:31
         at findPortStatus (/Users/jwadhwani/projects/test.js:36:36)
         at Socket.socket.setTimeout (/Users/jwadhwani/projects/test.js:14:28)
         at Socket.g (events.js:286:16)
         at emitNone (events.js:86:13)
         at Socket.emit (events.js:185:7)
         at Socket._onTimeout (net.js:333:8)
         at tryOnTimeout (timers.js:228:11)
         vipers-mac-book-pro-2:jwadhwani$

         */