ldc-doux/app/Http/Requests/UpdateSliderRequest.php

<?php

namespace App\Http\Requests;

class UpdateSliderRequest extends StoreSliderRequest
{
    /**
     * Get the validation rules that apply to the request.
     *
     * @return array
     */
    public function rules()
    {
        return array_merge(
            parent::rules(),
            [
                'slug' => 'required|unique:sliders,slug,' . $this->slider->id
            ]
        );
    }
}