Groups authors by author/name and returns all authors that have more than one distinct author/@id attribute

groupby.xq

let $authors := 
<authors>
  <author id="a"><name>Foo</name>…whatever…</author>
  <author id="b"><name>Foo</name>…whatever…</author>
  <author id="a"><name>Foo</name>…whatever…</author>
  <author id="c"><name>Bar</name>…whatever…</author>
  <author id="d"><name>Bar</name>…whatever…</author>
  <author id="f"><name>FooBar</name>…whatever…</author>  
</authors>
return
<distinct-names> {
  for $author in $authors//author
  group by $name := $author/name
  return
    if(count(distinct-values($author/@id)) > 1) then
      element {"author"}
              { 
                attribute {"name"} {$name},
                for $id in distinct-values($author/@id)
                return <id>{$id}</id>
              }
        else ()  
}</distinct-names>