A literal output element with two attribute sets listed in xsl:use-attribute-set receives a merged set of attributes from both sets. 

Should the same attribute appear in both sets, the one from the set mentioned last in xsl:use-attribute-set should win. This is not what xsltproc does, and I think this is a bug.

Core of the sample:

  <xsl:template match="a">
    <b xsl:use-attribute-sets="att1 att2"/>
  </xsl:template>

  <xsl:attribute-set name="att1">
    <xsl:attribute name="att1">1</xsl:attribute>
    <xsl:attribute name="commonatt"><xsl:message>Applying @commonatt from att1 set</xsl:message>v1</xsl:attribute>
  </xsl:attribute-set>

  <xsl:attribute-set name="att2">
    <xsl:attribute name="att2">2</xsl:attribute>
    <xsl:attribute name="commonatt"><xsl:message>Applying @commonatt from att2 set</xsl:message>v2</xsl:attribute>
  </xsl:attribute-set>

Result from xsltproc:
$ xsltproc a.xsl a.xml
Applying @commonatt from att1 set
<?xml version="1.0"?>
<b att1="1" commonatt="v1" att2="2"/>

Expected result (correct from saxon 6.5.5):
$ java -jar ~/lib/saxon.jar a.xml a.xsl 
Applying @commonatt from att1 set
Applying @commonatt from att2 set
<?xml version="1.0" encoding="utf-8"?>
<b att1="1" commonatt="v2" att2="2"/>

Note: not only the wrong value is chosen for @commonatt, but also the side effect is skipped by xsltproc.

The defect is present in (OS X):
$ xsltproc --version
Using libxml 20900, libxslt 10128 and libexslt 817
xsltproc was compiled against libxml 20900, libxslt 10128 and libexslt 817
libxslt 10128 was compiled against libxml 20900
libexslt 817 was compiled against libxml 20900,

as well as in (Linux):
$ xsltproc --version
Using libxml 20901, libxslt 10128 and libexslt 817
xsltproc was compiled against libxml 20901, libxslt 10128 and libexslt 817
libxslt 10128 was compiled against libxml 20901
libexslt 817 was compiled against libxml 20901

Ratio: https://www.w3.org/TR/xslt#attribute-sets: "Specifying a use-attribute-sets attribute is equivalent to adding xsl:attribute elements for each of the attributes in each of the named attribute sets to the beginning of the content of the element with the use-attribute-sets attribute, *in the same order* in which the names of the attribute sets are specified in the use-attribute-sets attribute." The spec regarding the order of addition is not satisfied.

Regards,
Mike