Help with Fatal error: Cannot redeclare _add_themes_utility_last

I am trying to install WordPress on a site for a client and I am stuck on the error message “Fatal error: Cannot redeclare _add_themes_utility_last() (previously declared in […]/wp-admin/menu.php:156) in […]/wp-admin/menu.php on line 157” when I visit my site’s /wp-admin/ link. When looking at the code, line 157 only has “}” on it. The block of code I believe it’s referring to (from lines 151 to 157) is

// Add 'Editor' to the bottom of the Appearance menu.
if ( ! is_multisite() )
	add_action('admin_menu', '_add_themes_utility_last', 101);
function _add_themes_utility_last() {
	// Must use API on the admin_menu hook, direct modification is only possible on/before the _admin_menu hook
	add_submenu_page('themes.php', _x('Editor', 'theme editor'), _x('Editor', 'theme editor'), 'edit_themes', 'theme-editor.php');
}

I cannot seem to find a resolution in the WordPress forums or from Google. Any help is much appreciated!