delete from unique_channels
where exists (
  select * from unique_channels u2
  where unique_channels.frequenz <> u2.frequenz
    and unique_channels.ChannelsConfID = u2.ChannelsConfID
    and (select count(distinct ChannelsConfID) from unique_channels u3
         where u3.frequenz = unique_channels.frequenz) <
        (select count(distinct ChannelsConfID) from unique_channels u4
         where u4.frequenz = u2.frequenz)
);