DOCBOOK-APPS: Limited number of <question>s allowed.

I'm using FOP 0.20.4, XSL stylesheets 1.53 and Saxon 6.5.2.

So far everything works reasonably well, until I want more than six
<question>s in my document - going to pdf.  Up to six works fine, but if I
add one more I get the following error:

[ERROR] The id "d0e360" already exists in this document

In the fo, however, this id actually exists only once.  I get the same
error whether I put the questions inside <qandaentry> or not, and even if I
put two or more <question>s inside a <qandaentry>.  It doesn't seem to
matter if I use more <qandadiv>s or <qandaset>s.  Once I get more than six
<question>s it won't even finish parsing the file.  Is there some other
trick?

Thanks,
Merce