Exactly why you should use the re-name or .htaccess method.
I was thinking that too and was going to suggest that Google would never find the duplicated default.php because nothing is linked to it, but it looks like "Catalog" in the bread crumb trail is linked to default.php.
The require method will just ensure a page is brought up when you type in the address to your catalog without specifying a file. Clicking any links will then take you to default.php.
Catalog in the breadcrumb trail is linked to whatever you have defined FILENAME_DEFAULT to be in application_top.php. No links in the core code (or in any contributions come to that) should be hardcoded to point to default.php :shock: