在您的注册功能registration_function中进行

user App\User;
public function registration_function(Request $request)
{
  $user_db = User::all();
  $user_entered = $request->all();

  if(in_array($user_entered, $user_db)){
      Session::message("User already entered please login");
      return redirect()->route('login');
  }else{
      $this->validate($request, [
      'email' => 'required|unique:registered|max:255',
      'password' => 'required',
     ]);

    User::create($request);
  }

}

希望这会有所帮助.