coref generates duplicate ID --- bug?

I am using the coref element in versions 1.61.3 of the docbook 
stylesheets. The HTML translation
works fine. But the FO translation gives me a FOP error. Looking at the 
FO file, the stylesheet is
generating duplicate fo:inline elements with the same id attribute 
values, one for the <co/> and one for
the <coref/>. Is this legal FO?

So is this a bug with the 1.61.3 stylesheet to produce these? Or is this 
perfectly file FO and a bug in FOP
to fail to handle duplicate IDs correctly?

Dave Bridgeland