Error message when trying to register a user - "The email has already been taken"

I have been following the following tutorial: https://www.toptal.com/laravel/restful-laravel-api-tutorial, however I have an issue.

I am up to the stage where I run the following curl code in my terminal:

curl -X POST https://myapp.jdoe.blah.test/api/register \
 -H "Accept: application/json" \
 -H "Content-Type: application/json" \
 -d '{"name": "Elle", "email": "[email protected]", "password": "total123456", "password_confirmation": "total123456"}'

According to the tutorial, I should receive the following information in this form, of course with my details instead of his:

{
    "data": {
        "api_token":"0syHnl0Y9jOIfszq11EC2CBQwCfObmvscrZYo5o2ilZPnohvndH797nDNyAT",
        "created_at": "2017-06-20 21:17:15",
        "email": "[email protected]",
        "id": 51,
        "name": "John",
        "updated_at": "2017-06-20 21:17:15"
    }
}

However instead I get the following message:

{"message":"The given data was invalid.","errors":{"email":["The email has already been taken."]}}

I checked in my users table and the user was successfully registered, but why am I receiving this message? Also, in the users table, the api_token field is empty.

The code in RegisterController is:

<?php

namespace App\Http\Controllers\Auth;

use App\User;
use App\Http\Controllers\Controller;
use Illuminate\Support\Facades\Hash;
use Illuminate\Support\Facades\Validator;
use Illuminate\Foundation\Auth\RegistersUsers;
use \Illuminate\Http\Request;

class RegisterController extends Controller
{
    use RegistersUsers;

    /**
     * Where to redirect users after registration.
     *
     * @var string
     */
    protected $redirectTo = '/home';

    /**
     * Create a new controller instance.
     *
     * @return void
     */
    public function __construct()
    {
        $this->middleware('guest');
    }

    /**
     * Get a validator for an incoming registration request.
     *
     * @param  array  $data
     * @return \Illuminate\Contracts\Validation\Validator
     */
    protected function validator(array $data)
    {
        return Validator::make($data, [
            'name' => ['required', 'string', 'max:255'],
            'email' => ['required', 'string', 'email', 'max:255', 'unique:users'],
            'password' => ['required', 'string', 'min:8', 'confirmed'],
        ]);
    }

    /**
     * Create a new user instance after a valid registration.
     *
     * @param  array  $data
     * @return \App\User
     */
    protected function create(array $data)
    {
        return User::create([
            'name' => $data['name'],
            'email' => $data['email'],
            'password' => Hash::make($data['password']),
        ]);
    }

    protected function registered(Request $request, $user)
    {
        $user->generateToken();

        return response()->json(['data' => $user->toArray()], 201);
    }
}

The code in api.php:

Route::post('/register', 'Auth\RegisterController@register');

The code in User.php:

<?php

namespace App;

use Illuminate\Notifications\Notifiable;
use Illuminate\Contracts\Auth\MustVerifyEmail;
use Illuminate\Foundation\Auth\User as Authenticatable;
use Illuminate\Http\Request;

class User extends Authenticatable
{
    use Notifiable;

    /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $fillable = [
        'name', 'email', 'password',
    ];

    /**
     * The attributes that should be hidden for arrays.
     *
     * @var array
     */
    protected $hidden = [
        'password', 'remember_token',
    ];

    /**
     * The attributes that should be cast to native types.
     *
     * @var array
     */
    protected $casts = [
        'email_verified_at' => 'datetime',
    ];

    public function generateToken()
    {
        $this->api_token = str_random(60);
        $this->save();

        return $this->api_token;
    }
}

I am new to Laravel, so if you need me to provide code for something else, please say and I will.

Thanks in advance :).