Fixed a problem when saving imported Categorizations

ccm-core/src/main/java/org/libreccm/categorization/CategorizationImExporter.java

@@ -70,7 +70,7 @@ public class CategorizationImExporter
     @Override
     @Transactional(Transactional.TxType.REQUIRED)
     protected void saveImportedEntity(final Categorization entity) {
-        entityManager.persist(entity);
+        entityManager.merge(entity);
     }
 
     @Override