code: private static void checkDuplicates(ImmutableList<SQLPPTriplesMap> mappings)
              throws DuplicateMappingException {
  
          Set<SQLPPTriplesMap> mappingSet = new HashSet<>(mappings);
  
          int duplicateCount = mappings.size() - mappingSet.size();
  
          /**
           * If there are some triplesMaps, finds them
           */
          if (duplicateCount > 0) {
              Set<String> duplicateIds = new HashSet<>();
              int remaining = duplicateCount;
              for (SQLPPTriplesMap mapping : mappings) {
                  if (mappingSet.contains(mapping)) {
                      mappingSet.remove(mapping);
                  }
                  /**
                   * Duplicate
                   */
                  else {
                      duplicateIds.add(mapping.getId());
                      if (--remaining == 0)
                          break;
                  }
              }
              //TODO: indicate the source
              throw new DuplicateMappingException(String.format("Found %d duplicates in the following ids: %s",
                      duplicateCount, duplicateIds.toString()));
          }
      }
docs: No mapping should be duplicate among all the data sources.