2.2.8 [W3C-XSLT] Section 7.1.4, Named Attribute Sets


The specification states:

 It is an error if there are two attribute sets that have the same expanded-name and
 equal import precedence and that both contain the same attribute, unless there is a
 definition of the attribute set with higher import precedence that also contains
 the attribute. An XSLT processor may signal the error; if it does not signal the
 error, it must recover by choosing from amongst the definitions that specify the
 attribute that have the highest import precedence the one that was specified last
 in the stylesheet.


No error is signaled. Instead, the last attribute that occurs in the style sheet is chosen.