Editing content type causes duplication of menu record in DB

Hi, first things first:

- Drupal: 5.1
- Configuration file: Protected
- Cron maintenance tasks: Last run 44 min 44 sec ago
You can run cron manually.
- Database schema: Up to date
- File system: Writable (public download method)
- GD library: bundled (2.0.28 compatible)
- MySQL database: 4.1.15
- PHP: 4.4.4-0.dotdeb.3
- Unicode library: PHP Mbstring Extension
- Web server: Apache/1.3.33 (Debian GNU/Linux) mod_layout/3.2 PHP/4.4.4-0.dotdeb.3 with Suhosin-Patch mod_ssl/2.8.22 OpenSSL/0.9.7e

Not sure whether this is a bug or not, but I've found out that after editing a content type (administer > content management > content types > edit), an extra record gets created in the menu table.

So, changing the value for "type" (e.g. from image to images) results in a extra record in the menu table (instead of a changed record - the thing that happens when another value gets changed). The extra menu line is not visible in the admin interface though, just in the DB table menu.