<xsl:if test="current() = ../ALIAS[1]"><h5>ALIASES: </h5></xsl:if>

  This is equivalent to "see if the current node does not have any preceding
  siblings that are also called ALIAS".  In other words:

  <xsl:if test="not(preceding-sibling::ALIAS)"><h5>ALIASES: </h5></xsl:if>

  I don't know which gives better performance.

They test different things. the first tests if the string value of the
current node's content is equal to the string value of the first ALIAS

The second tests if there is an earlier ALIAS node.

David


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