PREFIX skos: <http://www.w3.org/2004/02/skos/core#>
PREFIX : <http://purl.org/net/schemarama#>
CONSTRUCT
{
  [] a :Error;
    :message 'Literal [1] is both preferred and alternative lexical label for concept [2].';
    :implicated ( ?l ?x );
  .
}
WHERE
{
  ?x skos:prefLabel ?l.
  ?x skos:altLabel ?m.
  FILTER ( str(?l) = str(?m) && lang(?l) = lang(?m) )
}