PREFIX skos: <http://www.w3.org/2004/02/skos/core#>
PREFIX : <http://purl.org/net/schemarama#>
CONSTRUCT
{
  [] a :Error;
    :message 'Resource [1] has more than one preferred lexical label [2][3] in a given language.';
    :implicated ( ?x ?l ?m );
  .
}
WHERE
{
  ?x skos:prefLabel ?l; skos:prefLabel ?m.
  FILTER ( str(?l) != str(?m) && lang(?l) = lang(?m) )
}