code: def assert_pks_uniqueness(self, pks, exclude, value):
          
          pks = list(set(pks))
          if len(pks) > 1:
              # this may not happen !
              raise UniquenessError(
                  "Multiple values indexed for unique field %s.%s: %s" % (
                      self.model.__name__, self.field.name, pks
                  )
              )
          elif len(pks) == 1 and (not exclude or pks[0] != exclude):
              self.connection.delete(self.field.key)
              raise UniquenessError(
                  'Value "%s" already indexed for unique field %s.%s (for instance %s)' % (
                      self.normalize_value(value), self.model.__name__, self.field.name, pks[0]
                  )
              )
docs: Check uniqueness of pks
  
          Parameters
          -----------
          pks: iterable
              The pks to check for uniqueness. If more than one different,
              it will raise. If only one and different than `exclude`, it will
              raise too.
          exclude: str
              The pk that we accept to be the only one in `pks`. For example
              the pk of the instance we want to check for uniqueness: we don't
              want to raise if the value is the one already set for this instance
          value: any
              Only to be displayed in the error message.
  
          Raises
          ------
          UniquenessError
              - If at least two different pks
              - If only one pk that is not the `exclude` one