Laravel Check Email and Provider Exist

Question

I'm using laravel and I want to make auth register and checking by email and provider. Use email and provider because users can register if users login with social media (ex:facebook) have same email like users login with email. I can only make checking by email. this is my Register Controller

<?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;

class RegisterController extends Controller
{

    public function __construct()
    {
        $this->middleware('guest');
    }


    protected function validator(array $data)
    {
        return Validator::make($data, [
            'name' => 'required|string|max:255',
            'username' => 'required|max:255',
            'email' => 'required|string|email|max:255|unique:users',
            'password' => 'required|string|min:6|confirmed',
            'provider' => 'unique:users',

        ]);
    }

    /**
     * Create a new user instance after a valid registration.
     *
     * @param  array  $data
     * @return \App\User
     */

    protected function create(array $data)
    {
        // $prov = 'Email';

        return User::create([
            'name' => $data['name'],
            'username' => $data['username'],
            'email' => $data['email'],
            'password' => Hash::make($data['password']),
            'provider' => 'Email',
        ]);
    }
}