DOCBOOK-APPS: Duplicate ID in FO-File

[Peer Brink <brink@riege.de>]

After transforming DocBook-XML-document with DOCTYPE SET and two BOOKs (see example below) to FO, the resulting FO-document 
has a duplicate ID! FOP reports
"The id "toc...N400006" already exists in this document"
and the pdf-file is broken.

I am using stylesheets v. 1.50.0. Is it a stylesheet-bug?

Example
-------
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE set PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN" 
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd";>
<set>
  <title>TEST-SET</title>
  <book>
    <chapter>
      <title>BOOK 1</title>
      <section>
        <title>Section-Title BOOK 1</title>
        <para>Para BOOK 1</para>
      </section>
    </chapter>
  </book>
  <book>
    <chapter>
      <title>BOOK 2</title>
      <section>
        <title>Section-Title BOOK 2</title>
        <para>Para BOOK 2</para>
      </section>
    </chapter>
  </book>
</set>