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.
MSXML3 and MSXML6
No error is signaled. Instead, the last attribute that occurs in the style sheet is chosen.