Node.jsでhttpサーバー起動時にError: listen EADDRINUSE: address already in use :::8000が出る

vagrant@ubuntu-bionic:~/workspace/intro-curriculum-3011$ node index.js
events.js:167
      throw er; // Unhandled 'error' event
      ^

Error: listen EADDRINUSE: address already in use :::8000
    at Server.setupListenHandle [as _listen2] (net.js:1290:14)
    at listenInCluster (net.js:1338:12)
    at Server.listen (net.js:1425:7)
    at Object. (/home/vagrant/workspace/intro-curriculum-3011/index.js:13:8)
    at Module._compile (internal/modules/cjs/loader.js:689:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10)
    at Module.load (internal/modules/cjs/loader.js:599:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
    at Function.Module._load (internal/modules/cjs/loader.js:530:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:742:12)
Emitted 'error' event at:
    at emitErrorNT (net.js:1317:8)
    at process._tickCallback (internal/process/next_tick.js:63:19)
    at Function.Module.runMain (internal/modules/cjs/loader.js:745:11)
    at startup (internal/bootstrap/node.js:282:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:743:3)

// 起動中のプロセスを確認
$ vagrant@ubuntu-bionic:~/workspace/intro-curriculum-3011$ ps aux | grep node
vagrant   4528  0.0  3.1 561376 31868 pts/1    Sl+  21:22   0:00 node index.js
vagrant   6842  0.0  0.1  14164  1088 pts/0    S+   21:56   0:00 grep --color=auto node

// killでプロセスを終了させる
//   うまく終了しないときは"kill -s 9 プロセスID"
vagrant@ubuntu-bionic:~/workspace/intro-curriculum-3011$ kill 4528

// きちんと終了したかどうかをチェック
vagrant@ubuntu-bionic:~/workspace/intro-curriculum-3011$ ps aux | grep node
vagrant   6861  0.0  0.1  14164  1056 pts/0    S+   21:59   0:00 grep --color=auto node

vagrant@ubuntu-bionic:~/workspace/intro-curriculum-3011$ node index.js
Listening on 8000