code: public boolean duplicatesInStack(LinkedList<SchemaTableTree> distinctQueryStack) {
          Set<SchemaTable> alreadyVisited = new HashSet<>();
          for (SchemaTableTree schemaTableTree : distinctQueryStack) {
              if (!alreadyVisited.contains(schemaTableTree.getSchemaTable())) {
                  alreadyVisited.add(schemaTableTree.getSchemaTable());
              } else {
                  return true;
              }
          }
          return false;
      }
docs: Checks if the stack has the same element more than once.
  
  @param distinctQueryStack
  @return true is there are duplicates else false