how to avoid creating a slug that already exists as a Route "Symfony 4"

I have the entity Page identified by slug. Also I have the action to view a page in the Page controler :

class PageController extends AbstractController{   /**    * @Route("/{slug}", name="fronend_page")    */   public function show(Page $page)   {       return $this->render("font_end/page/show.html.twig", ["page" => $page,       ]);   }}

I am looking for good practice to validate the slug ( check if exist in routes) before save it in the database without use prefixesExample :

route exist : @route ("/blog")

check if blog exist before create slug : /{slug} = /blog

thanks