code: def _dedup_index(self, df_a):
          
          pairs = self._link_index(df_a, df_a)
  
          # Remove all pairs not in the lower triangular part of the matrix.
          # This part can be inproved by not comparing the level values, but the
          # level itself.
          pairs = pairs[pairs.labels[0] > pairs.labels[1]]
  
          return pairs
docs: Build an index for deduplicating a dataset.
  
          Parameters
          ----------
          df_a : (tuple of) pandas.Series
              The data of the DataFrame to build the index with.
  
          Returns
          -------
          pandas.MultiIndex
              A pandas.MultiIndex with record pairs. Each record pair
              contains the index values of two records. The records are
              sampled from the lower triangular part of the matrix.