site_icon.php

<?php
// To be used by theme authors to avoid duplicate favicon code now that Site Icons have been added to WordPress 4.3
// If a Site Icon hasn't been set or if the `has_site_icon` function doesn't exist (ie older than WordPress 4.3) 
if ( ! ( function_exists( 'has_site_icon' ) && has_site_icon() ) ) {
  // your theme specific custom favicon code goes here
}