2.1.5 [XML] Section 3.1, Start-Tags, End-Tags, and Empty-Element Tags
The specification states:
Well-formedness constraint: Unique Att Spec An attribute name MUST NOT appear more than once in the same start-tag or empty- element tag.
MSXML3 and MSXML6
An attribute can appear more than once in the same start-tag or empty-element tag. However, the first declaration takes precedence and the subsequent declarations are ignored.