Need to Test for Duplicate Slugs

For this kind of test you need to avoid using factory to know what to expect (in case you encounter a bug one day).

1. Create a post with manual data
2. Create an array containing key/values representing your columnName/value pairs in your post model. Make sure these values differ from the first post, except for the slug (so it is easy to distinguish).
3. Call your store route and pass your array to it.
4. Assert that the second post wasn't inserted

That would be the simple way, but you could also replace the 4th step by retrieving the count of the posts having the specified slug and assert that the count is 1

It's also important to note that unit tests should test your code (validation of the store route in your case) and not pre-built functionalities such as the database unique constraint or the factory implementation (except if you want to test that your factories are still up to date as you grow and add column to the model).

Would something like this maybe work?

/** @test */
    public function a_post_cannot_have_a_duplicate_slug()
    {
        $post = factory('App\Post')->raw();

        $this->post('/blog', $post)
            ->assertRedirect('/blog');

        $post2 = [
            'title' => $this->faker->sentence,
            'body' => $this->faker->paragraph,
            'slug' => $post['slug']
        ];

        $this->post('/blog', $post2);

        $this->assertDatabaseMissing('posts', $post);
    }

Yes (even though I'm not a fan of faker in unit tests like I said). But you would have to change $post by $post2 in your assertion.

Small mistake on the last line.

You want to assert you're missing $post2 , not $post

$this->assertDatabaseMissing('posts', $post2);

edit: oops didn't refresh the page see @piljac1 answer haha