code: public ClassificationModel attachLink(ClassificationModel classificationModel, LinkModel linkModel)
      {
          for (LinkModel existing : classificationModel.getLinks())
          {
              if (StringUtils.equals(existing.getLink(), linkModel.getLink()))
              {
                  return classificationModel;
              }
          }
          classificationModel.addLink(linkModel);
          return classificationModel;
      }
docs: Attach the given link to the classification, while checking for duplicates.