Clones of built in content types?

First of all: thank you very much for this powerful and awesome module.

Here is my request:

I just managed to export a book content type and to import it again.

First time importing it I overwrote the book content type with bookpage. Then I started another attempt and finally managed to create a second book type and a third (docbook and programme_page). They all behave like book pages and I can add some fields to one type of them and other fields to other types - EXACTLY what I wanted to do :)

I managed to do this by editing the type/old_type/orig_type settings.

I cannot reproduce the same behaviour on an other site I am running with CCK 5.x-1.3. This, and the fact, that I cannot delete the content types I once cloned makes me stop and think.

Am I using some new undocumented feature? Or did I stumble upon a bug?

If you are not aware about this behaviour, could you give me a hint to how to find out? What part of CCK is responsible for node cloning?

I would be glad if you could shed some light on this...

Alex