Is the ID attribute of type ID or NMTOKEN ?

While working with Arbortext Editor I copied an element containing an ID. In the copied element the ID appeared unchanged and the result of the validation was "no completness error found".

That was unexpected so I checked the langref.

The langref tells that the ID attribute is of type ID, so it should be invalid when I have the same ID twice. 

Then I checked the id-atts in the commonElements.mod and found:
id       NMTOKEN      #IMPLIED

So I guess the description in the langref telling 
id       ID           #IMPLIED
is wrong, but why.

I would expect id as being of type ID?

What is right?

Best regards

Chris