DOCBOOK: Uniq values of "ID" attributes

Hello!

1.
I'm join modular DocBook/XML documents and get errors for already
declared values of ID attributes (each in separated documents).

It's possible to do common attribute "id" uniq not for all 
document, in some levels only ?

2.
It's possible to add <fallback> element within <xref> element ? 
And also for all idref's attributes?

This may be another way to linking between documents, which 
located in one set of documents.

PS On page
http://www.docbook.org/tdg/en/html/ch01.html
exist small text error in first para:
"you only need to skim this chapter."
probably must be:
"...to skip..."

-- 
Regards, Vyt
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