1. preceding-sibling:

Instead of <xsl:if test="self::node()[1]"> use <xsl:if
test="not(preceding-sibling::field/@id = @id)">

won't this return the very first field in document order?

No, it only tests whether there is not a preceding-sibling field element, which has the same @id like the current one.

You can write "not(preceding-sibling::field[@id = current()/@id])" if you want.

It is my understanding that if you have:

<a>
  <f id="1"/>
  <f id="2"/>
  <f id="2"/>
</a>

and you happen to be on the 3rd f node you would get:

preceding-sibling::field/@id=1

and current()/@id=2

but if you use:

preceding-sibling::field[1]/@id=2

Is this wrong?