Are 'du' and 'df' reliable when there are lots of hard links?
From man du:


  Files having multiple hard links are counted (and displayed) a single time per du execution.
  
  Directories having multiple hard links (typically Time Machine backups) are counted a single time
       per du execution.


From man df:


  The df utility displays statistics about the amount of free disk space on the specified
       filesystem or on the filesystem of which file is a part.


So assuming both man pages are correct neither utility should count hard linked things twice.