EDIT

    public TaxonNode addChildTaxon(Taxon taxon, int index, Reference citation, String microCitation) {

        Classification classification = HibernateProxyHelper.deproxy(this.getClassification(), Classification.class);

        taxon = HibernateProxyHelper.deproxy(taxon, Taxon.class);

            throw new IllegalArgumentException(String.format("Taxon may not be in a classification twice: %s", taxon.getTitleCache()));

       }

       return addChildNode(new TaxonNode(taxon), index, citation, microCitation);