Is it legal to do <xsl:copy-of select="element/@attri

Is it legal to copy attributes that are collected from a bunch of
different elements into a result element using
  <xsl:copy-of select="element/@attribute"/>?

It works in the processors I've tested, but as the same attribute may
occur several times in the selected node set, but can only be present
once in the output, I'm not sure whether it's O.K. to do something
like this (in XSLT 1.0).  Can I rely on the rule for <xsl:attribute/>
being applied that says, when adding an attribute, any already
existing attribute of the same name is replaced?  Or is the processor
behavior implementation-dependent in such a situation?

Thanks!
Thomas W.