PHP/Laravel:为什么要调用这个函数?

我想了解为什么此函数在laravel中被调用两次,我定义了此工厂:

<?php

use Faker\Generator as Faker;

function my_callback (Faker $faker) { // <= LINE 5 
    return [
        // some key-value pairs, don't focus on that
        'id_ubicacion' => 1,
        'nombre' => $faker->name,
        'clase' => 'CONTROL ADMINISTRATIVO',
        'codigo' => $faker->ean13,
        // some other key-value pairs
    ];
} // <= LINE 29

$factory->define(App\Bien::class, 'my_callback');

然后,我进行了以下测试:

<?php

namespace Tests\Unit;

use Tests\TestCase;
use Illuminate\Foundation\Testing\WithFaker;
use Illuminate\Foundation\Testing\RefreshDatabase;

use Faker\Factory as Faker;

class BienTest extends TestCase
{
    use RefreshDatabase;

    public function test__actualizar()
    {
        // setup
        $registro_a_actualizar = factory('App\Bien')->create(['id' => 1]);

        // more code...

    }

    public function test__destruir()
    {
        // setup
        $registro_a_destruir = factory('App\Bien')->create(['id' => 1]);

        // some code...
    }

当我执行 单元测试 我得到以下错误

Fatal error: Cannot redeclare my_callback() 

(previously declared in \inven\database\factories\FactoryBien.php:5) 

in \inven\database\factories\FactoryBien.php on line 29

my_callback() 是否不止一次被呼叫?