DU and hard links

I have a series of backup home directories what use hardlinks on files
that have not changed.   If I run "du" on these directories I get
a disk usage summery as if the directories are not hard linked together!

How can I get real disk usage summery.  IE: the cost in disk space of
the second and later directories without re-counting files already seen
due to hard links!!!

  Anthony Thyssen ( System Programmer )    http://www.cit.gu.edu.au/~anthony/
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               A novice practices until he gets it right.
            An expert practices until he doesn't get it wrong.
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     Anthony's Home is his Castle     http://www.cit.gu.edu.au/~anthony/