PREFIX skos: <http://www.w3.org/2004/02/skos/core#>
PREFIX : <http://purl.org/net/schemarama#>
CONSTRUCT
{
  [] a :Warning;
    :message 'Resource [1] has more than one stated preferred symbolic label [2][3] - are these the same resource?';
    :implicated ( ?x ?n ?o );
  .
}
WHERE
{
  ?x skos:prefSymbol ?n; skos:prefSymbol ?o.
  FILTER ( ?n != ?o )
}